A good portable awk shebang is not easy to find.

shebang.awk

#!/usr/bin/awk -f
#!/usr/bin/awk -E
#
# If you have an awk version that doesn't support the -f flag,
# then you are just out of luck.
#
# If you just have no clue where awk will be, or you prefer to use -E,
# then you can try this bash snippet to launch awk for you.
#
# You might think the -E tests below are overly complex. You'd be wrong.
# I thank Apple for their wonderfully broken awk on Mac OS X.
# If you find a system, this doesn't work on please let me know!
#
# Copy the below line to the top of the file:
#!/usr/bin/env bash
#
# Inspired by: https://unix.stackexchange.com/a/97280
#
_src="${0}"
function exec_awk() {
  _cmd="$(command -v awk)"
  _awk_prog="/awk:/ && /option/ && /-E/ {exit 1;} {next}"
  _flag="-$("$_cmd" -E /dev/null </dev/null 2>&1 | "$_cmd" "$_awk_prog" && echo 'E' || echo 'f')"
  true bash#; exec "${_cmd:-false}" "${_flag}" "${_src}" "$@"; exit;
}

true {}#; set -e; exec_awk "$@"

# You place your awk program below this line and make sure this file has
# execute permissions.

The goal is to have awk run the content of the file without introducing additional dependencies. (Using bash is a fallback that's not preferred.)

As the comments say, if you already know where awk will be and/or which variant of awk will be used, and you don't care about supporting any unknown systems, then just write the shebang you need using that knowledge.