If one were to be a one-issue voter (not recommended, btw), #ClimateChange seems like the only that would make sense as the one-and-only thing to care about.
Nothing else compares to the scale and magnitude of suffering it will cause. No other issue has litteral world-ending consequences (except, possibly, a nuclear war).
When one candidate is still actively denying climate change and the other is pushing for progress to combat it, at least a little, the choice should be obvious.