Prime Ali vs prime Tyson would probably be the most financially lucrative......and exciting in the count down. It'd probably be a good fight to watch too.
But for sheer excitement in the ring you need two guys with aggression, punch power, decent chins (you wouldn't want a quick fire KO!) and a fair disregard for defense:
Prime Foreman vs Prime Liston would be good!
As would Tyson vs Marciano