I give it to Black Mage if he is sleeping mobs when I am tank, healers if they are doing a great job of keeping me alive and depending on how things go it might go to dps for doing a great job of picking up adds.
If I am DPS (Archer) I will give it to another dps or the healer. However, if the tank is super nice and giving other players helpful instruction, marking mobs, controlling argo and just being a kick <snip> tank you will get my vote for sure.

I am really torn when I get a bunch of noobs in a dungeon and they all do a great job. I usually say I want to give one to everyone but unfortunately you can't. It would be nice if you could do a commendation for everyone once a week for those special times.