Mega Man 2
Featuring more robot masters, a password system, and slightly better box art, every aspect of Mega Man 2 is an improvement over the original game.
If you ask most Mega Man diehards what the best game in the series is, they're almost guaranteed to come back with either MM2 or MM3. 2 seems to hold a slight edge, though. It's the one featured on the NES Classic, and it's the game that shows up on all the "greatest games of all-time" lists. It's a phenomenal video game that's as fun to play today as it was 30 years ago.
I don't often get into a game's history or production notes, but the story of Mega Man 2 is just too great not to bring up. The original Mega Man game didn't sell overly well. However, instead of just writing the series off, Capcom allowed their staff to make a sequel, with a catch; they work on it off company time. That's precisely what they did, and using ideas that didn't make the cut for the first game, they ended up creating a masterpiece.
I love all the core Mega Man games (except 8. Fuck that game so hard), and excitedly replayed them all when the Mega Man collections came to the Nintendo Switch last year. I don't know if I could bring myself to definitively rank them, but I'd agree with almost everyone that 2 is among the best. the robot masters are awesome, and it features the single most powerful robot master weapon in the series' history; Metal Man's metal blades. Mega Man is one of my go to fighters in the newer Smash Bros games, and that so many weapons from this game made it into his arsenal for Smash is a testament to how awesome they were.
My buddy David Rae joined me this week, and we remembered one of the greatest games in one of the greatest series in video game history; Mega Man 2.