Gotta be Toughness. It's a weak feat at first level and a waste of a feat slot at any higher level. I've never known anybody to take core rules toughness for their character and changing Toughness to 1 HP/level seems to be one of the most common house rules out there.
Runners up: Combat Casting (why get a +4 bonus to one use of a skill when you could get a +3 bonus to all uses of the same skill) and Endurance (the addition of Die Hard makes Endurance slightly better in 3.5 but it still isn't too good, PC's dont' tend to get in forced march situations too often)
The +2 to 2 skills feats can be quite usefull, depending on the character build and skills in question. Not universally desierable but not the Worst Feat Ever by any stretch of the imagination.
Later.