Whoever did good dps for their role and job will get my comm. The only exception I'll make perhaps is for a healer who carries a struggling party or a tank with well timed LB (e.g. when too few alive for stack marker).

Usually if there's a standout dps (partly considering their position in aDPS meta) I'll give them the comm since often you can see a clear distinction, otherwise I'll give it to the best performing Tank or Healer dps wise.