The best Mega Man game made, in my opinion. Why they decided to make a gazillion crappy versions after this one is beyond me, though these days it seems like the developers are starting to respond to the cry of the fan boys, with releases like Mega Man 9 and Mega Man 2.5.
How has it stood against the test of time against the newer reiterations of the game? Great platforming game with the right balance of difficulty. For me, it was a game that you didn't get sick of. Back then, NES games that I liked were ones where I could go through within a day, without the frustrations of control issues and poor design decisions. It made it replayable because you didn't have to invest so much time in it, sort of like how Game Boy games are today.
Music was catchy. Design was fun. The game was about figuring out the patterns of your rivals (legendary), and taking advantage of "game flaws" like pressing start a zillion times at that one sub-boss stage...
No side jumping.
Just Mega Man.