Grovyle's sprite is AWFUL. I really like Grovyle, but as previously mentioned it looks like a dinosaur and VERY little like Grovyle. And what's with blue leaves?
Most of Ruby, Sapphire and Emerald's sprites are inferior to FireRed and LeafGreen's in terms of quality. In particular, Squirtle('s arms), Butterfree, NIDORINO, Primeape, Tauros (it's tail tips are lacking their blue colour), Jolteon, Flareon and Dragonair. If you compare them to FireRed and LeafGreen's hpefully you'll see what I mean.
Another thing I noticed which is fixed in FireRed and LeafGreen, is that R/S's (don't know about Emerald) Wigglytuff back sprite is lacking the white tips on it's ears.
Along with Nidorino who I mentioned earlier, the worst R/S sprite (at least out of the ones which got revamped) is Snorlax's. The face is completely wrong and it's hands only have 3 big 'fingers' as opposed to 5 small ones. It's sprite got completely re-drawn in Emerald though. I guess it was really that bad.
Most of the R/S (#252-386) Pokémon's sprites are quite bad too. Take Dusclops for example. It's missing a finger on both hands.