So here they are, in my opinion:
#1:regigigas
I refuse to capitalize regigigas because I hate it to death. I don't think this atrocity can be called a Pokemon. Seriously, what was GameFreak smoking when they decided to create this thing? I mean, COME ON.
1. Horrible ability: Slow Start makes you weak and slow for 5 turns, which I guess CAN be stalled in-game, but 5 turns is enormous in competitive battling, especially when regigigas doesn't have access to Protect or Rest.
2. Horrible name: regigigas? Really? And this is why most people call it "Regigigigigigigigigigigigigigigas".
3. Horrible design: what is it supposed to be? regigigas looks like a face with weird ears and a weird nose. And what's up with those green feet?
4. Extremely painful to catch: To get it, you will need to catch all 3 Regis from Emerald, then transfer them to Platinum and you can get regigigas. Sounds easy, right? Well, not really.
a) All 3 Regis have a catch rate of 3, so you'll need a lot of PokeBalls to get them, and possibly some soft resets because they just won't let themselves get catched.
b) You need to trade them to Platinum, which means you need 2 DS's.
c) regigigas is at level 1, which means you'll need to use a HM slave or a Pokemon with False Swipe.
d) regigigas still has a catch rate of 3, so you'll need once again to throw a lot of Dusk Balls, PokeBalls or whatever.
So yeah, regigigas is my absolute least favorite Pokemon of Gen 4.