Way back at the Dawn of Time, Mega Man 2 for the NES was declared the best 2D action game that will ever exist. Oh, you weren't present for that ceremony? Just take my word for it and download Mega Man 2 from the Nintnedo 3DS eShop. Doesn't matter if you own the original cartridge, or if you've already downloaded the game from the Wii's Virtual Console. Do it again. Play it again.
It's $4.99 USD.