Mine is probably the Nordic freeroll on
pokerstars. Or at least used to be. There are so many
freerolls its difficult to remember all of them.
The first deposit freeroll on Fulltilt was nice too, but I dont know if they have those for Americans.
I liked the first deposit $2500 freerolls they had on PokerStars before as well.
They were hilarious, everytime I raised everyone pretty much assumed I had the Aces.