Mega Man 2 is one of the best games ever because:
-Incredibly tight controls.
-Epic power ups (METAL BLADE!!!!!).
-Awesome levels and level design.
-Perfect gameplay pacing.
-Awesome game pacing.
-Endless replay value.
-Satisfying final boss, story ending, credits, and overall game ending.
-Most importantly, the game is about as much fun as can be fun in a video game!
The same is true (although IMO, to a slightly lesser extent) for Mega Man 1 and 3.