Mega Man X4

The formula does admittedly feel a bit tired but things are freshened up with a few interesting level designs such as going across the airships or through the constantly shifting cyber-world, and the play control is nice and smooth.

A return to form after X3 despite not adding anything new to the series aside from some cheesy cutscenes for a really dumb plot and a more consistently playable Zero. The game is also generally easier compared to X1-X2 imo. But the addition of Zero alone marks this as an innovation as Zero plays drastically different from X and the levels become a lot harder and more fun due to his playstyle. I wouldn't call this the peak of the Megaman series, but it's a really damn good title regardless.