My preferred Rubocop config

.rubocop.yml

AllCops:
  RunRailsCops: true

# Commonly used screens these days easily fit more than 80 characters.
Metrics/LineLength:
  Max: 120

# Too short methods lead to extraction of single-use methods, which can make
# the code easier to read (by naming things), but can also clutter the class
Metrics/MethodLength: 
  Max: 20

# The guiding principle of classes is SRP, SRP can't be accurately measured by LoC
Metrics/ClassLength:
  Max: 1500

# No space makes the method definition shorter and differentiates
# from a regular assignment.
Style/SpaceAroundEqualsInParameterDefault:
  EnforcedStyle: no_space

# Single quotes being faster is hardly measurable and only affects parse time.
# Enforcing double quotes reduces the times where you need to change them
# when introducing an interpolation. Use single quotes only if their semantics
# are needed.
Style/StringLiterals:
  EnforcedStyle: double_quotes

# We do not need to support Ruby 1.9, so this is good to use.
Style/SymbolArray:
  Enabled: true

# Most readable form.
Style/AlignHash:
  EnforcedHashRocketStyle: table
  EnforcedColonStyle: table

# Mixing the styles looks just silly.
# REVIEW: Enable once https://github.com/bbatsov/rubocop/commit/760ce1ed2cf10beda5e163f934c03a6fb6daa38e
#   is released.
#Style/HashSyntax:
#  EnforcedStyle: ruby19_no_mixed_keys

# has_key? and has_value? are far more readable than key? and value?
Style/DeprecatedHashMethods:
  Enabled: false

# String#% is by far the least verbose and only object oriented variant.
Style/FormatString:
  EnforcedStyle: percent

Style/CollectionMethods:
  Enabled: true
  PreferredMethods:
    # inject seems more common in the community.
    reduce: "inject"


# Either allow this style or don't. Marking it as safe with parenthesis
# is silly. Let's try to live without them for now.
Style/ParenthesesAroundCondition:
  AllowSafeAssignment: false
Lint/AssignmentInCondition:
  AllowSafeAssignment: false

# A specialized exception class will take one or more arguments and construct the message from it.
# So both variants make sense. 
Style/RaiseArgs:
  Enabled: false

# Fail is an alias of raise. Avoid aliases, it's more cognitive load for no gain.
# The argument that fail should be used to abort the program is wrong too,
# there's Kernel#abort for that.
Style/SignalException:
  EnforcedStyle: only_raise

# Suppressing exceptions can be perfectly fine, and be it to avoid to
# explicitly type nil into the rescue since that's what you want to return,
# or suppressing LoadError for optional dependencies
Lint/HandleExceptions:
  Enabled: false

Style/SpaceInsideBlockBraces:
  # The space here provides no real gain in readability while consuming
  # horizontal space in that could be used for a better parameter name.
  # Also {| differentiates better from a hash than { | does.
  SpaceBeforeBlockParameters: false

# No trailing space differentiates better from the block:
# foo} means hash, foo } means block.
Style/SpaceInsideHashLiteralBraces:
  EnforcedStyle: no_space

# { ... } for multi-line blocks is okay, follow Weirichs rule instead:
# https://web.archive.org/web/20140221124509/http://onestepback.org/index.cgi/Tech/Ruby/BraceVsDoEnd.rdoc
Style/Blocks:
  Enabled: false

# do / end blocks should be used for side effects,
# methods that run a block for side effects and have
# a useful return value are rare, assign the return
# value t to a local variable for those cases.
Style/MethodCalledOnDoEndBlock:
  Enabled: true

# Enforcing the names of variables? To single letter ones? Just no.
Style/SingleLineBlockParams:
  Enabled: false

# Shadowing outer local variables with block parameters is often useful
# to not reinvent a new name for the same thing, it highlights the relation
# between the outer variable and the parameter. The cases where it's actually
# confusing are rare, and usually bad for other reasons already, for example
# because the method is too long.
Lint/ShadowingOuterLocalVariable:
  Enabled: false

# Check with yard instead.
Style/Documentation:
  Enabled: false 

# This is just silly. Calling the argument `other` in all cases makes no sense.
Style/OpMethod:
  Enabled: false 

# There are valid cases, for example debugging Cucumber steps,
# also they'll fail CI anyway
Lint/Debugger:
  Enabled: false

May I gist this?

Where should I put this file?
In my home (~/) or the project's home?

The file config/default.yml under the RuboCop home directory contains the default settings that all configurations inherit from. Project and personal .rubocop.yml files need only make settings that are different from the default ones. If there is no .rubocop.yml file in the project or home directory, config/default.yml will be used.

https://github.com/bbatsov/rubocop/blob/master/manual/configuration.md#defaults

I thought I got it.

I don't like how it tells you to NOT put spaces before "when" and "else". How do I stop that?

case
  when 'bla'
    .....
  when 'aaa'
    ....
  else
    .....
end

Following is the configuration works with rubocop 0.52.0 after fixed warnings

Rails:
  Enabled: true

# Commonly used screens these days easily fit more than 80 characters.
Metrics/LineLength:
  Max: 120

# Too short methods lead to extraction of single-use methods, which can make
# the code easier to read (by naming things), but can also clutter the class
Metrics/MethodLength:
  Max: 20

# The guiding principle of classes is SRP, SRP can't be accurately measured by LoC
Metrics/ClassLength:
  Max: 1500

# No space makes the method definition shorter and differentiates
# from a regular assignment.
Layout/SpaceAroundEqualsInParameterDefault:
  EnforcedStyle: no_space

# Single quotes being faster is hardly measurable and only affects parse time.
# Enforcing double quotes reduces the times where you need to change them
# when introducing an interpolation. Use single quotes only if their semantics
# are needed.
Style/StringLiterals:
  EnforcedStyle: double_quotes

# We do not need to support Ruby 1.9, so this is good to use.
Style/SymbolArray:
  Enabled: true

# Most readable form.
Layout/AlignHash:
  EnforcedHashRocketStyle: table
  EnforcedColonStyle: table

# Mixing the styles looks just silly.
Style/HashSyntax:
  EnforcedStyle: ruby19_no_mixed_keys

# has_key? and has_value? are far more readable than key? and value?
Style/PreferredHashMethods:
  Enabled: false

# String#% is by far the least verbose and only object oriented variant.
Style/FormatString:
  EnforcedStyle: percent

Style/CollectionMethods:
  Enabled: true
  PreferredMethods:
    # inject seems more common in the community.
    reduce: "inject"


# Either allow this style or don't. Marking it as safe with parenthesis
# is silly. Let's try to live without them for now.
Style/ParenthesesAroundCondition:
  AllowSafeAssignment: false
Lint/AssignmentInCondition:
  AllowSafeAssignment: false

# A specialized exception class will take one or more arguments and construct the message from it.
# So both variants make sense.
Style/RaiseArgs:
  Enabled: false

# Indenting the chained dots beneath each other is not supported by this cop,
# see https://github.com/bbatsov/rubocop/issues/1633
Layout/MultilineOperationIndentation:
  Enabled: false

# Fail is an alias of raise. Avoid aliases, it's more cognitive load for no gain.
# The argument that fail should be used to abort the program is wrong too,
# there's Kernel#abort for that.
Style/SignalException:
  EnforcedStyle: only_raise

# Suppressing exceptions can be perfectly fine, and be it to avoid to
# explicitly type nil into the rescue since that's what you want to return,
# or suppressing LoadError for optional dependencies
Lint/HandleExceptions:
  Enabled: false

Layout/SpaceInsideBlockBraces:
  # The space here provides no real gain in readability while consuming
  # horizontal space that could be used for a better parameter name.
  # Also {| differentiates better from a hash than { | does.
  SpaceBeforeBlockParameters: false

# No trailing space differentiates better from the block:
# foo} means hash, foo } means block.
Layout/SpaceInsideHashLiteralBraces:
  EnforcedStyle: no_space

# { ... } for multi-line blocks is okay, follow Weirichs rule instead:
# https://web.archive.org/web/20140221124509/http://onestepback.org/index.cgi/Tech/Ruby/BraceVsDoEnd.rdoc
Style/BlockDelimiters:
  Enabled: false

# do / end blocks should be used for side effects,
# methods that run a block for side effects and have
# a useful return value are rare, assign the return
# value to a local variable for those cases.
Style/MethodCalledOnDoEndBlock:
  Enabled: true

# Enforcing the names of variables? To single letter ones? Just no.
Style/SingleLineBlockParams:
  Enabled: false

# Shadowing outer local variables with block parameters is often useful
# to not reinvent a new name for the same thing, it highlights the relation
# between the outer variable and the parameter. The cases where it's actually
# confusing are rare, and usually bad for other reasons already, for example
# because the method is too long.
Lint/ShadowingOuterLocalVariable:
  Enabled: false

# Check with yard instead.
Style/Documentation:
  Enabled: false

# This is just silly. Calling the argument `other` in all cases makes no sense.
Naming/BinaryOperatorParameterName:
  Enabled: false

# There are valid cases, for example debugging Cucumber steps,
# also they'll fail CI anyway
Lint/Debugger:
  Enabled: false

# Style preference
Style/MethodDefParentheses:
  Enabled: false

Thank you so much!

# Commonly used screens these days easily fit more than 80 characters.
Metrics/LineLength:
  Max: 120

# Too short methods lead to extraction of single-use methods, which can make
# the code easier to read (by naming things), but can also clutter the class
Metrics/MethodLength:
  Max: 20

# The guiding principle of classes is SRP, SRP can't be accurately measured by LoC
Metrics/ClassLength:
  Max: 1500

# No space makes the method definition shorter and differentiates
# from a regular assignment.
Layout/SpaceAroundEqualsInParameterDefault:
  EnforcedStyle: no_space

# Single quotes being faster is hardly measurable and only affects parse time.
# Enforcing double quotes reduces the times where you need to change them
# when introducing an interpolation. Use single quotes only if their semantics
# are needed.
Style/StringLiterals:
  EnforcedStyle: double_quotes

# We do not need to support Ruby 1.9, so this is good to use.
Style/SymbolArray:
  Enabled: true

# Most readable form.
Layout/HashAlignment:
  EnforcedHashRocketStyle: table
  EnforcedColonStyle: table

# Mixing the styles looks just silly.
Style/HashSyntax:
  EnforcedStyle: ruby19_no_mixed_keys

# has_key? and has_value? are far more readable than key? and value?
Style/PreferredHashMethods:
  Enabled: false

# String#% is by far the least verbose and only object oriented variant.
Style/FormatString:
  EnforcedStyle: percent

Style/CollectionMethods:
  Enabled: true
  PreferredMethods:
    # inject seems more common in the community.
    reduce: "inject"


# Either allow this style or don't. Marking it as safe with parenthesis
# is silly. Let's try to live without them for now.
Style/ParenthesesAroundCondition:
  AllowSafeAssignment: false
Lint/AssignmentInCondition:
  AllowSafeAssignment: false

# A specialized exception class will take one or more arguments and construct the message from it.
# So both variants make sense.
Style/RaiseArgs:
  Enabled: false

# Indenting the chained dots beneath each other is not supported by this cop,
# see https://github.com/bbatsov/rubocop/issues/1633
Layout/MultilineOperationIndentation:
  Enabled: false

# Fail is an alias of raise. Avoid aliases, it's more cognitive load for no gain.
# The argument that fail should be used to abort the program is wrong too,
# there's Kernel#abort for that.
Style/SignalException:
  EnforcedStyle: only_raise

# Suppressing exceptions can be perfectly fine, and be it to avoid to
# explicitly type nil into the rescue since that's what you want to return,
# or suppressing LoadError for optional dependencies
Lint/SuppressedException:
  Enabled: false

Layout/SpaceInsideBlockBraces:
  # The space here provides no real gain in readability while consuming
  # horizontal space that could be used for a better parameter name.
  # Also {| differentiates better from a hash than { | does.
  SpaceBeforeBlockParameters: false

# No trailing space differentiates better from the block:
# foo} means hash, foo } means block.
Layout/SpaceInsideHashLiteralBraces:
  EnforcedStyle: no_space

# { ... } for multi-line blocks is okay, follow Weirichs rule instead:
# https://web.archive.org/web/20140221124509/http://onestepback.org/index.cgi/Tech/Ruby/BraceVsDoEnd.rdoc
Style/BlockDelimiters:
  Enabled: false

# do / end blocks should be used for side effects,
# methods that run a block for side effects and have
# a useful return value are rare, assign the return
# value to a local variable for those cases.
Style/MethodCalledOnDoEndBlock:
  Enabled: true

# Enforcing the names of variables? To single letter ones? Just no.
Style/SingleLineBlockParams:
  Enabled: false

# Shadowing outer local variables with block parameters is often useful
# to not reinvent a new name for the same thing, it highlights the relation
# between the outer variable and the parameter. The cases where it's actually
# confusing are rare, and usually bad for other reasons already, for example
# because the method is too long.
Lint/ShadowingOuterLocalVariable:
  Enabled: false

# Check with yard instead.
Style/Documentation:
  Enabled: false

# This is just silly. Calling the argument `other` in all cases makes no sense.
Naming/BinaryOperatorParameterName:
  Enabled: false

# There are valid cases, for example debugging Cucumber steps,
# also they'll fail CI anyway
Lint/Debugger:
  Enabled: false

# Style preference
Style/MethodDefParentheses:
  Enabled: false

Rubo has renamed some fields. Here is the latest one without warning