String Zip and Sort

GS2, 4 bytes

*Ü■/

This reads the strings from STDIN, separated by linefeeds.

The source code uses the CP437 encoding. Try it online!

Test run

$ xxd -r -ps <<< '2a 9a fe 2f' > zip-sort.gs2
$ echo -e 'HELLO\nworld\n!!!!!' | gs2 zip-sort.gs2 
!Hw!Eo!Lr!Ll!Od

How it works

*       Split the input into the array of its lines.
 Ü      Zip the resulting array.
  ■     Map the rest of the program over the resulting array.
   /        Sort.

Haskell, 39 36 bytes

import Data.List
(>>=sort).transpose

Usage example: ((>>=sort).transpose) ["HELLO","world","!!!!!"] -> "!Hw!Eo!Lr!Ll!Od".

Transpose the list of strings, map sort over it and concatenate the resulting list of strings (>>= in list context is concatMap).

Pyth, 5 bytes

Zips(C) the input(Q), Maps Sort, then sums.

sSMCQ

Try it online.