The game where the Mega Man series really began, if you could only start with one game in the series, make it 2 and skip 1.
1 is miserable by comparison, and that's odd because structurally there's not much different. But as a Mega Man fan, 1 feels like obligation when you're playing Legacy Collection and 2 feels like actual fun.
With tightened up control, improved physics, better level design, fun weapons, and even better graphics and soundtrack than the first, Mega Man 2 blows the original away.
My only real problem are the 3rd and 5th Wily Castle Bosses. The third requires you to blow through all your Crash Bomb ammo and Game Over if you can't do it right on the first go.
And the fifth can only be hurt by your bubbles with, again, no way to regain your ammo so it's an instant Game Over.
Save states are a must in those cases but other than those late game hurdles, Mega Man 2 has stood the test of time.

If you look up "needs refinement" you'll certainly find the bizarre US boxart of the original Mega Man staring at you with his cold dead eyes.
The game's controls are both too heavy and too slippery, Mega Man himself takes a moment to either go or stop, and some of the level design is actual butt cheeks. Lookin' at you, Elec Man.
But you do see the trappings of brilliance underneath the poor controls and level design. Getting new powers is fun, the sound track is solid, and the graphics are great for 1987.
Mega Man is a solid enough game, but it's not the one I'd recommend you start the series with. Especially given the lack of healing items and poor control compared to later entries.