I think the race will be closer than many people think, even if Trump is the nominee.
With that in mind, I'd prefer Senator Kaine from Virginia. Virginia(13EVs) has been trending blue over the past decade, and he's stayed rather popular since going from being Mayor of Richmond to being Senator. That, and his Spanish is pretty decent. One undercovered story of 2015 was the number of appearances Hillary has made on Spanish-speaking media outlets, so he'd dovetail nicely with her.
The Vice Presidential debate in October also takes place right outside of Richmond. They'd be able to exit the debate and do one hell of a rally that night, or in the following days. Kaine lines-up with her so damn well.
Given that so many states have gone blue in every election since 1992, and given that New Mexico and Nevada are looking further and further out of reach for the GOP, locking-down Virginia would put her one state away from 270. Her opponent would be able to win Florida, Ohio, North Carolina, Iowa, Colorado.. if she wins lil' ol' New Hampshire(4EVs) - another state that's been trending blue - she'd be right at 270. Game over. We'd know the race is over when the East Coast polls are closed and VA is called.