"Oh yawn a Breloom.. Magic Bounce". I fear no sleep. The status I fear is burn induced scald. Even if a fire type can't be burned they still get super effective damage.
The burn from scald can't be magic bounced away, and its only counters are water absorb abilities, and magic guard(if this can be called a counter). Also there's the fact that it has incredibly high distribution.
Anyway, even though Sleep is a close second, I have to go with Badly Poisoned. Toxic is the main reason why Pokemon such as Blissey and Shuckle can actually do a lot of damage, and it could force out switches if the bad poison gets out of hand. Plus, Breloom and Gliscor benefit from the Toxic Orb if they have Poison Heal, which recovers 1/8th of their HP in addiction with preventing another major status. Facade also gets boosted by either bad poison or a Toxic Orb, becoming a great STAB move on Pokemon such as Ursaring and Swellow. However, I think a Flame Orb or getting a burn is slightly more useful for Facade than being badly poisoned is, due to the increasing HP loss you get when you use a Toxic Orb. Finally, there's Toxic Spikes, where if you set up it twice, it can become a big problem for the opposing team. Unless they have Rapid Spin.
Official Mike Meekins of the AA5 boards. Official Olimar of the SSB Wii U Boards. Playing: Psychonauts, Luigi's Mansion 2