worst standalone ability is probably either slow start or truant, but the only pokemon who have these are given massive BSTs to compensate. all things considered i think klutz is probably the worst up there.
Heavy Metal is blah. Better Heavy Slams, more damage from Low Kicks. Weak Armor actually has some use. Illuminate wastes your time and is useless. Look on the bright side! More shiny chances! Klutz is terrible, but it just isn't THAT bad. You can still function with it. In addition, all Pokémon that get it get another ability. Stall…? I've not heard of this before xP 2nd worst ability in the game I think. Run Away keeps you from being trapped by Dugtrio. Truant cripples Slaking, but Durant can use it with Entrainment to exploit the AI and go hundreds of battles into the Subway. Slow Start takes what would otherwise be an Uber and yanks it down to the nether-regions of NU. It gets my vote. Defeatist hurts a lot, but Archeops was meant to live fast and die young.
Why do I have to move with the crowd of kids that hardly notice I'm around? I work myself to death just to fit in.