The Dustin-Conor one stands out among most other mentions in this thread so far because I think it's the best example of a rematch that demonstrated change. You always have your instant rematch examples or borderline fluke KO examples, where the rematch is just another way the first fight
could have gone, but didn't. But for Dustin-Conor, I mean, you
might make the argument that DP's varied skillset and superior groundwork could have led him to a victory when they first fought, but I don't think anyone can argue DP could have lit Conor up like he did in the following matches.
So while I know a bunch of people are satisfied by rematches that seem to right some perceived wrong, my favorites are the one that display just how different things have become.
Another example I think of is Condit-Kampmann. The Dane was a solid stalwart to debut against, and it was a real mixed, back and forth, extremely close fight. 4 years later, there was a stark difference, and it lasted
a while
I'm sure there are much better examples, but that's just the one on my mind
*edit
Now that's a perfect example