Tennis is my favorite. It just feels so good, so smooth and responsive. And it's great for four players. The girls and I have been having a blast with it. I've been getting fairly good at it, but I have been playing it more than any of the other games. I'd look forward to a future Mario Tennis game, if one was ever to come around. Such an enjoyable game.
Bowling would be my second favorite, but that might change once I have more exposure with the others. I have a feeling I'll enjoy boxing way more once I get the learning curve down a bit more. I just need to take the time to do it, but Tennis has been too much fun!
I hear the true depth of the game comes from the Training Mode, but I haven't even been able to try it out yet. I guess they add obstacles to bowling and more pins and stuff like that.
Boxing hands down. Punching someone in the face has never been more fun.
Tennis by a huge margin. Boxing's terrible, man! Bowling's fun too, but it's actually quite challenging to not get a strike.