I'll go with luvdisc.
Other candidates I thought about:
Sunkern - Pokeathlon. (Even though some don't care, others do)
Magikarp - Not useless since it evolves into Gyarados. Could be worst, but Gyarados isn't.
Farfetch'd - Even though it can't battle, it still looks awesome.
Dunsparce - Actually has pretty decent stats, and has room for development. (Normal/Dragon evo, anyone?)
Unown - I used to diss on unown, but not anymore after that horrendous experience. (If you get frightenend easily, you don't want to know.)
Delibird - It is really badass in manga.
Plusle + Minun - They're useless, but I think it does have potential to be better if developers cared.