How many angels fit on the head of a pin?
None of these candidates really had anywhere near a shot at winning - so, debating who has a better shot is pointless. The only way we get to any candidate defeating any of those three incumbents is through major scandal or death or something that could be applied to ANY of the candidates.
Sure, a more intense sex scandal in '96 could've sunk Clinton. But it being revealed Reagan knowingly had Alzheimer's in 1984, but hid it, could have been just as devastating. Or the economy doesn't rebound at near as fast of a rate because, for a bevy of reasons, like Paul Volcker not being kept. Reagan was vulnerable in 1982. If things continue to stagnate, he's toast in '84. With Nixon, it could be Watergate and the complete meltdown he had in the wake of it.
I mean, my point is that any devastating event to shift one election could be used, in other ways, to shift any of the other elections. So, saying 'if Clinton had a sex scandal in '96 Dole could've won' is really no different than saying 'if Reagan didn't do this or that early in his presidency, Mondale could've won'. They're both plausible in the realm of what we're discussing, but also so radically divergent from reality that it kind of makes this whole point moot.