Moore/Robinson, which came close enough to getting made.
Then, coming to mind are:
Tyson/Holy, 88-89, soon after Holy's move up, within a few fights.
McCallum/Hearns, at... 154...? 160?
Mosley/Trinidad, at 147.
Pacquiao/Mayweather, 2009-2010, at welterweight. I'm not even one of either man's bigger fans, but it would've been a fight to settle the biggest bout of both casual and hardcore boxing fans' arguments of the past decade. ...Or ended in a horrific and obscene controversial decision that causes it to be the biggest bout of arguments of this decade too.
...Maybe it's just as well. Forget I mentioned them and replace this one with Jose Sulaiman VS Sergio Martinez.