Okay, I can't decide, so I'll list my opinions.
Sleep - ugh. I hate being unable to do anything, but at least I wake up a bit later.
Paralysis - I don't mind the speed drop, as I usually use slow pokemon, and it's great with Gyro Ball :). But when I'm fully paralyzed, that's just irritating. Once, my Regirock got fully paralyzed for 15 turns in a row.
Confusion - This is probably why I use defensive pokemon. If you have high attack and low defense, you end up suiciding.
Freeze - not much of a problem, as I don't face it that much, but when I do, it's worse than sleep. I do tend to freeze my opponents, so I guess they should be the ones complaining.
Burn - the bane of physical sweepers. It also hurts you every turn, but not outside of battle like poison.
Poison - that out-of-battle damage is really annoying, thank god they removed that in B/W. But it's still annoying when you have to switch out constantly to prevent Toxic from becoming too strong.
Attract - every time my pokemon get infatuated, I can't attack. or even use healing moves. or anything. Genderless pokemon ftw, though.
and the most forgotten one, Curse - Normally, not a big problem, as you usually end up OHKOwning the ghost, and you can switch out. But if you're doing a solo run, you're dead. I hated Morty in my solo run, feels like I couldn't win.