Probably Lapras
I mean I still like the design and it's not a bad pokemon, but I used to think it was this amazing almost quasi-legendary powerhouse back in Gold, this was before I had any idea of stats, physical/special moves, STAB...basically anything.
I didn't really get back into pokemon until Black/White, though played a bit of the previous gens, think I had a go on a friends or something. But I became aware of the competitive scene, not so much wanting to actually be involved, but realising that pokemon had these stat spreads, STAB and such mattered a great deal, EV training existed, natures etc. I realised Lapras just wasn't that great, particularly offensively, pretty much mediocre stats all round bar its high hp and horribly slow. Didn't have any decent abilities, especially if you weren't running a rain team. Just...wasn't what I thought it was.
Then as others have said Pigeot, back in the day it was fine but now it's just so obviously outclassed by so many other flying types, sad really..