I used to think Stamin-Up was a waste of a perk slot, but recently I've grown fond of it and consider it in my 4 perks.
When it comes to my second least favorite though, it has to be Mule Kick. Yep guys, Mule Kick. Mule Kick has really hurt me far more than it has helped me, since when I get into a tight situation where I need to act quickly, I usually forget the order of the guns and swap ot the wrong one, or waste time cycling through them. I generally only need two weapons anyways.
Worse than Mule Kick is Tombstone. Worst perk ever IMO. Typically you don't have an adequate team to spawn you near it, and you have to waste a life to actually use it. Not to mention it isn't even available on solo.