Worst lens would have to be the EF 90-300. Utter peice of crap, I only used it for some sport shots when the trusty 70-200mm f2.8 was in for repairs.

Best lens would have to be the Minolta 58mm f1.2 used it on a film slr body when I was learning but never really appreciated it until i got an adapter for my 5dmkII. the 24-70 f2.8 and the 70-200 f2.8 (non IS) are also awesome lenses which have served me incredibly well.