At first glance, I'd have gone with Baltimore, since they have like a full 5 man rotation of young guys either up already, or on their way in Matusz, Tillman, Arrieta, Britton, and Erbe, as well as a young core in Wieters, Jones, Pie, Reimold, Markakis, etc.
However, considering that they have to contend with the Yankees, Sox, and Rays in the AL east, it kind of dims their future a little bit.
Texas has a whole boatland of talent out there in the AL West. Feliz, Holland, Smoak, Davis, Borbon, Martin Perez etc. Hard to bet against them having big things in their future.
In the NL, it has to be the Marlins, as usual. They already got a fairly young core in Johnson, Nolasco, Hanley, et al, as well as Morrison and Stanton on the way up.