Sorry for the MAJOR bump, but many users weren't around when this thread was made and I figured it would be better to bring this one back, rather then to make a new one.
Anyway my personal favorite is Smooth Moves, I think it really took advantage of the Wiimote with it's different form controls and is just the one I tend to go back to the most. With that said I also love Mega Party Game$ for it's multiplayer, which I prefer over Smooth Moves' multiplayer.
The only game in the series I'm not too hot on is Snapped, other then that I like them all, even the (understandably) contested Game & Wario.
Game and Wario. It's mostly due to Ashley's game, that was fun and I spent a good amount of time getting the highest scores I could get to the point of getting every small secret and not getting hit once not missing a thing and doing my best to keep the orbs. Good times.
I voted for WarioWare: Touched! mostly cause of my experience with it. I remember bringing it along with me when I was forced to head out somewhere. The mini games and survivors would keep my young mind entertained for a long time
It's a hard choice between Touched and Twisted, but I went with Touched because it introduced Ashley >.< I played Touched and Twisted the most out of the series. I remember Twisted having the best batch of the Pyoro-esque side-games, and I played those for a looong time.
I really loved Touched! Considering it was one of the first games on my DS. I really need to buy it back now! I haven't played any other wariowares unfortunately. However, I also remember doing that birthday microgame and getting so good I got to 699 on that one. I was so lightheaded xD