I would have to say Corkscrew. Not only is is not a very fun inversion, if it is on any Vekoma/Arrow coaster, it bashes your head. That = No fun.
I've got a headache even from B&M corkscrews. I don't really like them, even in Inverteds they pull too much blood to your feet. Goudurix of Parc Asterix would've been a fun coaster if it wouldn't have been for the corkscrews[V]. They are the worst inversions by far I think.
Loops are very cool inversions, as are