Confusion and infatuation are both merely annoying, because it's easy to get rid of. You change your Pokémon, confusion or infatuation gone!
Poison, paralysis and sleep are my most hated status ailments. Poison lingers after battle, and if you haven't got an Antidote with you, you'll either be using Potions to keep your Pokémon alive, or it just faints. Paralysis is just like confusion an infatuation, but more powerful. It lingers after battle, and it cuts your Pokémon's speed. I think sleep is my single most hated ailment. I'm fighting a strong Pokémon, and my Pokémon falls asleep. And you'd think it would be shook awake after taking a hit, but no... even a Hyper Beam won't wake you up! Before your Pokémon wakes up in battle, it might already have fainted.
So, sleep is definetly my most hated status ailment.