Tyson / Ali would be the top spectacle I can think of. Might not end up the most exciting fight, or the best in a pound for pound sense, but I can't imagine a bigger one.
I'd debate given the popularity of boxing in Rockys time him vs Ali might generate more if they had the same level of exposure Tyson did.
Thomas Hearns vs Julian Jackson. Once at 154 and once at 160. Holyfield vs Frazier. Barrera vs McGuigan at feather. Azumah Nelson vs Manny Pacquiao at feather. Hatton vs Gatti. Trinidad vs Norris at 154.
Just the first of those Seamus. I know who wins the last three. We were only supposed to pick one and I've made my pick 13 years ago in this thread...........Ali / Louis. Time flies ! But if I had a second pick, it would be Holy, on the juice if he wants to, v Marciano.