All of them! ;P
No seriously, I love having new things and more special events available through events. With the advent of wifi, things are much more common than they used to be, and cloning (though it does have its negatives and some people don't consider it legitimate) has made many more event Pokemon available to many people that wouldn't have had them otherwise. Plus, you could probably even borrow the Pokemon from someone and give them back, provided you were trustworthy.
Aaaanyways, the one I'm probably looking forward to most is Arceus. It's a real shame that it's level 100 so it can't be properly trained, but I love being able to get one of the DPPt legendaries at level 1, and I'm a total sucker for special moves, so that makes this one even better! I'll just have to make sure I can trade to get them all... hee hee.