With Julian Castro as a vice presidential nominee, I'd say Texas.
Though it will become hard if Jeb Bush ends up as the Republican nominee. Missouri is drifting hard towards the Republican side, so it won't be easy to reverse that trend. The Dakotas, and especially North Dakota, are also trending Republican. Hmm, I think I'll end up with Texas actually, although it's much more likely to change sides in 2020. Rumour says that latino share of the Texas electorate (the actual voters who went to the polls I mean) increased from 20% in 2008 till 25% in 2012. That sounds like an insane increase from one election to another. Yet if that's the fact, it might as well increase further up to 30% or thereabouts in 2016. And close to 35% in 2020. In that case, all bets should be off.