Yang and Gabbard win going away, as they pull 10%+ of Trump voters away from him.
Harris is donezo, she's polling below 5% now, trending down.
Sanders would be a tough fight because the GOP would just say "socialism" like its a spooky word and half the country would get scared.
Warren is also a tough fight, mainly because I think Trump kills her in debates. He's already got her labeled and it was a bad gaffe on her part.
Biden I think wins, though not as easily as the polls indicate, mainly because yeah, I don't think he's good in debates. He sticks his foot in his mouth a lot. But he's popular where he needs to be as far as getting electoral votes, and as long as he doesn't make a critical error in debates, I think he would win. Maybe not too easily, but I think electorally Biden is probably in the best shape.
Obama also was in the middle of a recession when he took over. Trump has been riding on the coattails of Obama fixing the economy Bush ruined thus far. We'll start to see any impact Trump has shortly.