Slaking

With an Attack stat higher than that of Groudon, Zekrom, and Rayquaza, a respectable Speed tier, and a beefy base 150 HP stat, combined with great coverage moves such as Earthquake and Night Slash, Slaking looks like a top-tier Uber threat. That is, until you spot its ability, Truant, probably the single worst ability in the entire game. Because of Truant, Slaking becomes huge setup fodder for dangerous sweepers. It also keeps Slaking from having any way of sweeping the opposing team, even late-game. With such a letdown of an ability, Slaking is rather challenging to use. That said, it can and often will KO at least one of the opponent's team members if it gets a free switch in.