there are three answers to the main poll question:
for best ability you go with Alessandro Nesta. who when he was on, was unplayable. I mean absolutely phenomenal. dude just made defending look like a stroll, but he could also be an action-man CB if he needed to. his performances vs. us in the 2006 CL SF and united in the 2007 CL SF just made my head explode with defensive magnificence. but he's had a few years of injury-plagued awfulness, especially in the latter half of the decade.
for the best consistency, you go with Rio Ferdinand. he's had a lot of injuries basically in the last year, but in the 9 previous ones he was an utterly magnificent defender. he was never as good as Nesta individually, and for the most part he played with much worse team-mates (when United got him a CB partner near his level they tea-bagged the hell out of europe) but until that back issue he was 8/10 minimum every single week. now he's over the back problem you can see the effect his return has on united.
for the best success, you go with Carles Puyol. while Nesta was the most skilled defender and Rio the most consistent, neither has had the all-encompassing success that Puyol has managed. Nesta has 2 CL's, like Puyol, however he lacks the international pedigree that our Captain brings to the frame. Puyol was a central CB in 2 CL's, 3 lligas, 1 copa, a european championship AND a world cup. he's performed to an incredibly high standard in the biggest of big matches for club and country, and if oleguer could mark at set-pieces his teams wouldn't have conceded a single goal in the 4 major finals they played in.
Lucio is basically runner-up in the last two categories, so deserves an honourable mention at least.