I've seen Trump vs Clinton arguments made, but I am unsure if either is a viable candidate.
Many are calling for DeSantis on the right, but he may or may not try to run. Ted Cruz might run in the primaries, and may stand a chance to be selected, but so much of the left hates him that he's be as inviable as Trump in the general election. In the end, I think Trump still has enough grip on the Republican party to get nominated over someone who could have a better shot of winning overall.
I believe at least Clinton, Warren, Harris, and Bloomberg will run in the primaries if Harris is not selected to run as incumbent. Andrew yang will probably run for the party he formed (Progress Party), not as a Democrat. I think Clinton may have the best chance despite being a killer because only the right cares about that.
In the end, it will probably be some random senator or rep. from the Democrats who can claim to be a moderate and appeal to independents as being far removed from Biden despite being run by the same party. John Doe vs. Trump :/