Gotta be Pedroia. Youk makes a case, but he's a 30 year old corner infielder. Ultimately you can replace most of his value from free agency. I know I'll get a huge argument if I suggest this, but I feel about the same way about Lester. A second baseman who can put up a .850-.900 OPS consistently is a much rarer bird.
To put my argument a bit more distinctly, there are more corner infielders as good as Youkilis or better, and more lefthanded starters as good as Lester or better, than there are second baseman as good or better than Pedroia (Chase Utley's the only guy I can think of that's CLEARLY better, possibly Ben Zobrist if you made him a 2B but he's more a True IF/OF. Cano's too up and down, Uggla's batting average kills his chances, Kendrick, Hill, Weeks and Kinsler have arguments but also some serious flaws compared to the little big man)