option1.pp

class foo(
  $bar,
  $baz   = true,
  $gronk = false,
) {
  # ...
}

option2.pp

class foo(
    $bar,
    $baz   = true,
    $gronk = false,
  ) {
  # ...
}

option3.pp

class foo($bar, $baz = true, $gronk = false) {
  # ...
}

I put the curly brace on its own line, without indentation, which is closest to 2 I guess. Like shaftoe, Option 3 can be used when the number of parameters is small (3-4 max) and it fits in 80 columns.

Option 1 is how most of us PS folk will do it (with a space between the class name and curly brace)

Like @shaftoe here at @camptocamp, but I usually go for option 1 lately, regardless of the number of arguments.