That depends on what you like and how you define "fun". For instance, over the years I've had the best time with Baldur's Gate 2, but if you don't like/have the patience for RPG's that's no good. For laugh-out-loud funny, I'd say Ratchet & Clank 2 and 3 - fun to play as well, but not very challenging. And lately, I've found that Saint's Row 2 is more fun than a game should rightly be allowed to be; it's one of the few games I actually feel like PLAYING AROUND in, not just beating. But if you don't like the genre, it's probably not fun at all.
So it all is a matter of taste/interests/personality/available consoles.