Best overall player is probably Mauer (maybe Choo). But I already have Posey. Drafting a 2B is tempting because Utley's status is unclear. But I think I'd be better off targeting a guy like Kelly Johnson, Figgins, or Neil Walker later in the draft. Beltre is the last good 3B. Reyes's 2009 still scares me (and the Met's will probably suck this year). Overall, I'm not excited about any of the players available. Maybe it's just me.
I'd actually be tempted by Kershaw, Upton or Pedroia (assuming that's Justin). I think he's due for a bounceback, and I just don't think Mauer is worth it given his health history, low power, and the fact that you have Posey.
Of all of those, Upton is the sexy pick. Kershaw could give you a dominant start to a staff with your other guys there. Depending on how much good pitching is to follow.
I'm not interested in Mauer as you already have Posey. As for your 2B backup should Utley get injured again, I'd look for a guy like Walker later in the draft. Choo is rock-solid, but J Upton's ceiling is much, much, higher.
Given that you have C, 1B, 2B, OF, SP, SP kept, I would target J Upton. Snagging Kershaw would give you a brutal 1-2-3 of Kershaw, Hamels, and Jimenez, but with the other two already locked down you can afford to target some SP3s later on (Nolasco, B Anderson, T Lilly, etc..).
J Upton is a proven talent and still very young, hitting in a nice ballpark, and with more confidence. I'd draft him first.