What makes a great win (IMO) is a combination of factors. Obviously, upsets are in as of themselves great wins by definition.

But for me, it involves things like the skill level of both involved. Clashes between undefeated fighters are special, because one of them is going to suffer his first defeat. Along with the record itself, resumes have to be taken into consideration. It's not the same thing to have a 40-0 record fattened by taxi cab drivers, than a 20-0 record against increasingly tough opposition. Also I like all things to be equal. No weight draining... no (or few) special concessions to either fighter... and basically fighters at their peak.

I love fights with ebb and flow. That's why to me Leonard-Hearns I and Ali-Frazier III are in a category of their own. Back and forth and back and forth. With each fighter taking turns dominating the fight. Fighters changing strategy or game plan mid-fight in order to adjust.

Rivalry fights are special to me. Close to home, the PR-Mexico rivalry (in my obviously biased opinion ) is one of the best. But there's others as well.

I guess I would add historical significance, as it is a factor that cannot be overlooked. Some have already been mentioned. Along that vein, I would add those fights where records (meaningful ones) are set.

Finally... I just like fights where pride is at stake. Whether it's a rivalry, a personal thing... or whatever. I abhor fights that only represent efforts to achieve meaningless or stained trinkets.