I have a few I use
BOOOOOOOOOOOOOOOOOOOOM is my default
I also enjoy a good
www.OWNED.YOU or sometimes
www.OWNED.FISH
and a good
THX FISH is nice as well.
Most of these I just yell in IRC and not in the chat window.
If somebody underplays their hand, and like has JJ and flat called preflop and never raise on a Ten high board and beats my KT, I will always let loose a "bwahahahahaahahahahahah" and then not explain what happened.
If I was Asian- I would totally say "AYIAAAAAAAAAAAAAAA"