+1 for Ronnie Brewer (I've been touting him as the best wing defender in NBA for a couple years now) Such lateral quickness, tenacity while still being extremely long and tall.
Westbrook kills on D, and don't forget Iguodala (when he does defend) can strip that ball from you like nothing!
I would say Shane Battier hes been consistently the one best, the fact that hes with the rockets and that we havent gone deep into the playoffs hurts him, but its not his fault he brings it night in and night out, its our stars (yao/mcgrady) who have been hurt almost half of their carreers here in houston. Hope fully this season is different.