I say Gen 1 because if graphical limitations prevented them from making them too ornate. Though, special mention hours to Conkeldurr of Gen V. The is no other Pokemon I simply refuse to use due to appearance.
5th gen only really had Mienshao, Zorua, and Serperior that I really liked. There were a few "ok" ones like Samurott.
The rest are just downright hideous. Darumaka, what the **** are you supposed to be even?
Call it hideous all ye like, that's fine, but it does have a pretty clever design.
I voted gen 4 by the way. I like every gen, but that one had some really big stinkers with Rhyperior, Lickilicky, Magmortar, Probopass, the starters, Toxicroak, Bastiodon, Purugly and quite a few others. It's probably to blame on the fact that pretty much every ugly Pokemon got an evolution in this gen, thus generating a huge amount of ugly creatures.
They're on our left, they're on our right, they're in front of us, and they're behind us. They can't get away this time!