Environment. I’ve voted for the strongest environmental candidate ever since I was allowed to vote. Not just because I care about it, but because human industry and other land uses are driving non-human species to extinction, and causing problems that are difficult or impossible to reverse, and all life on the planet is interconnected, and so future generations will be more and more miserable and closer to our own extinction the more we keep doing this.
But also, more and more in recent years, corruption, meaning being in the pocket of big corporate/industrial interests, which includes almost everyone in both major parties. It tends to have me vote the same way (pro-industry tends to mean anti-environmental protection anyway), except that now I strongly prefer the pro-environment candidate who seems least corrupt in this way.
I have other issues as well, but those are the strongest. Often it’s very clear who I’ll vote for, though sadly in recent years and especially during the 2016 primary, it’s become clear that there are quite a few Democrats who are mostly pro-environment and try to act progressive, but then vote for corporate interests on key issues, and/or endorsed Clinton over Sanders.