Here's Youppi! (You have to use the exclamation point.)
I don't really have a favorite, the teams I'm a fan of don't NEED mascots. Can you imagine a Yankees mascot? I do however, HATE Mr. Met and Sparky, the New York Islanders mascot. Sparky actually is the mascot of the New York Dragons, an ARENA LEAGUE FOOTBALL TEAM. He just changes colors and jerseys for hockey season. Completely cheesy. But then, so are the Islanders.
Here's Sparky's stupid homepage:
http://www.sparkythedragon.com/
-Mikey