I am pretty certain one of the worst superpowers has to be the ability to regenerate and/or survive inflicted injuries. Why exactly though?
The reasoning is simple. Though you are effectively invincible, that doesn't necessarily equal the same thing - you're still going to feel everything and the fact that you can't die is going to lead you into more and more situations where you're going to get REALLY badly hurt. Not to mention, you're probably willing to take even bigger risks and get even MORE hurt in the knowledge that you can't die.
Worse yet, if a super-villain hooked you up to an endless torture machine, you'd be in quite a damn spot, now wouldn't you? Yes, regeneration/guaranteed-survival is nifty, but it doesn't come along with inherently offensive abilities like other powers and means that you're always 'the guy' that's going to get his ass kicked (whether by your whim or otherwise) over and over and over.