I would say a sinkhole. Falling into a volcano would probably be the most painful, (depending on how you went in) but I imagine falling into a sinkhole would be scarier. It could happen at any moment, and you would barely have time to process what's going on before you're already several metres underground. There's also the possibility of getting stuck and dying a slow and painful death before you can be rescued.
I feel like, for all the other options, you would either die on impact or be knocked unconscious and subsequently drown.