My favorite .COM-file still:

X5O!P%@AP[4\PZX54(P^)7CC)7}$EICAR-STANDARD-ANTIVIRUS-TEST-FILE!$H+H*

(yes, you can just copy-paste it into a .COM-file and it will execute and just show a text - unless you've got a correctly working virus scanner)

This is a Virus Scanner test signature (https://en.wikipedia.org/wiki/EICAR_test_file), so the fact alone that you can read this comment might be meaningful ;)