I've only played the first 4 so far, but so far Mega Man X is the best game in that entire series and it might end up being the best X game outright. To that end, it might be one of the finest run-and-gun type games ever made.
It evolved the Mega Man formula in a handful of ways, such as the boss gauntlet changing from the standard Mega Man warp room to a series of encounters within a continuous action setpiece of a level, which later games reneged on for the sake of maintaining the aforementioned formula.
2020 was the first time I got to sit down with a controller and play Mega Man X, and it was the first time I ever agreed with the hype. Mega Man X has aged like wine, keeping up with an entire industry of indie successors and possibly rising above every other entry in its home series. And it's held onto that reputation for almost thirty years.
I haven't played the last half of the X series, but as it stands, Mega Man X hasn't been topped by any of the games that followed it - not in its own series, and I would argue not even within its own genre.