If we're talking about unstaged fighting, then some are obvious:
Ford over Cleveland. Ford was an athlete and a Navy man. Cleveland was, frankly, kinda fat. He has a significant edge in weight, but he's shorter and lacks power. No contest.
Jackson over Lincoln. Both of these guys were skinny and sickly as president, but Jackson was a much tougher guy with a military background. Also a known fighter.
Obama over JFK. 'Cause JFK was as sickly as US presidents come, and Obama wasn't. Honestly, it seems pretty likely that Kennedy's team of doctors convince him to forfeit the bout.
Teddy over Wilson. 'Cause he's Teddy. Duh.
LBJ over Nixon. Bigger. More direct and aggressive personality. Has everything going for him.
The others are tougher to call. Clinton/Bush could definitely go either way. Similar size and physical conditioning if we give Clinton for his mid-term fitness, after he became scared to die and got into shape. Same goes for Papa Bush against Washington. Both tough older guys in decent condition for their age and of similar size and apparent strength. Bush has a little weight on Washington, but GW seems wiry. The exact opposite problem occurs in Trump/Reagan - too old, too little conditioning. Winner is whoever doesn't have a heart attack in the 3rd round.