MY BAD!!!! I really like the game's design – all the levels are super fun to go through and really hard, so there's a big incentive to learn the stage layouts and tactically take out enemies as you progress. Unlike a lot of later NES Mega Man games, it's never just moving forward and shooting – you have to make on-the-fly decisions about where you're going, where you're shooting, and why, and I find that really rewarding. The Yellow Devil is a fucker (though I have beaten him without the pause glitch

) and the Magnet Beam thing is really dumb, but I still have a lot of fun playing the game.
Also, and this is a minor thing, but I just really love how the entire game is clearly themed around construction. All the levels are job sites, and all the robot masters are actual utility machines for laying brick, incinerating material, demolition, etc. – I find that more interesting than just "THIS ONE'S THE WATER GUY!!!!" or "THIS ONE'S THE TIME GUY!!!" (They kind of ruined this in Powered Up, and I don't like that version too much.) It's a very unique theme for a video game, especially at that time, but it really makes the game stand out.
I don't really think MM1 is the best one – MM3 is probably what I'd recommend most – but it
is the one I go back to the most, and I love it.