2016 with Obama running*:
2008 with Bush running (though I doubt he would):
2000 with Clinton running:
1988 with Reagan:
*=Accidentally gave ME 1 to Trump instead of 2.
I think Obama and Reagan could both lose to the right candidate, Obama had pretty lackluster approvals until the very end of his term and I don't think he'd have seen the improvement he saw if he were to run again. As for Reagan, he was noticeably frail at the end and I thus factored it in the result. Without the frailty, Reagan likely would've met or exceeded Bush's real life result against someone like Dukakis.
I think Bush would've declined even if he were able to run. He had a tough Presidency and he seemed pretty eager to go back to Texas. I just made a map for the hell of it.
Clinton would've beaten just about any potential Republican candidate, scandal or not. I think someone like McCain would've given him a genuine run for his money and made it close, but I think in the end Clinton would've eeked it out. Regardless, he'd have fared worse electorally than he did in 1992 or 1996, but ironically might've gotten 50+% of the popular vote, something he never did, with the absence of Perot.