Is there any game or series that has better boss fights than the Mega Man franchise if playing buster only?
I can't think of a single game or series that beats them not just on quality and quantity but even also how the very idea of boss fights should look and function in a game as well. In a way I like to think that the franchise even invented the very concept of how to do proper boss fights.
All great boss fights in gaming should be challenging pattern based fights that test the limits of the player and force them to have to get good. All great games should also have a variety of different unique bosses to fight. There are plenty of game series that do this well of course but none as well as most of the Mega Man franchise. The one negative I'll give though is that the boss weakness in many of the games does destroy them too well but that is why I said if buster only. In some like X4 the right weapon destroys them pretty bad but others like NES MM3 the bosses can still be pretty tough. In a way the weapons kind of function like a built in easy mode crutch for the player but you can always also play the more challenging path and not use the weapons for at least most of the games.
So basically, when playing buster only these games have the best boss fights in all of gaming and more importantly, are boss fights done right.