Derek Jeter's 4 World Series rings.
EDIT: I didn't have time to comment on this earlier (and obviously Varitek is far from being my favorite player), but I am also very impressed by his no-hitter record. Especially considering that the last two were by very young pitchers (Buchholz and Lester), who likely depended a lot on his pitch-calling. It really shows how much this guy knows about baseball. And I hope he stinks it up for the rest of the year =P.
I should probably also add in Don Mattingly, who is my all time favorite player. I'm most proud of the fact that Donnie played for the Yankees his entire career (usually as their best player) during their longest post-season drought. Weird, huh? But, his play, his professionalism and his integrity always made me proud to be a Yankee fan no matter how much they disappointed me during all those years.