New Hampshire. If Clinton wins New Hampshire, it's pretty much over. Unless you think Trump can pull out a miracle win in Colorado (very doubtful).
But note that if she wins New Hampshire, she can lose pretty much every other true battleground state. We're talking Nevada, Ohio, North Carolina, Florida, AND Iowa.
I've never before seen a small state so important strategically (it's only worth 4 EVs). The campaigns seem to be aware of this and are still dumping resources in to win it's meager prize. Polls have been tightening there too. So before people get complacent, keep in mind that Clinton can win the popular vote by 5% and still lose this thing if she loses all the battlegrounds including NH.
But if NH goes for Clinton, you can go to bed early. If not, we're going to be in for a late night nailbiter that might hinge on Nevada, her last best hope if she loses NH and all of the other battlegrounds.