Onix - Less attack than a pidgey. Less health than a foetus. Can be one-shot by pretty much any attack except a normal-type move that nobody would use on it anyway. It's a shame because he's been my favourite for ages, but he's so goddarned bad.
Regigigas - Obviously, Slow Start. Totally cripples it, irredeemable.
Archeops - Oh, you got hurt? Would be a real shame if you lost half your stats into the bargain. Oops.
Unown - If you can find a use for these guys besides arranging them to spell "Fuck" in duels, you're a smarter person than me.
Abomasnow - As a kid, I found this intimidating. I was like "Oh, god, it's gonna kill me!" But then you breathe on it and it just dies.