I'm pretty torn between 3 or 6.
1 feels pretty outdated. The high score system, the item drops, the sound effects, the graphics...
2 is okay. Some of the graphics feel outdated by NES Mega Man standards. Metal Blade is dumb. Personally, I think this is very overrated because it's the first Mega Man for a lot of people.
3 is great. The extra bosses/stages are HARD but are nice additions. The introduction of slide makes the gameplay feel much much faster.
4 introduced charged shots, though it still felt soft here. Probably because Megaman glows green. Some nice hidden weapons.
5 had the best feeling charged shots out of all NES Mega Man titles. It really feels powerful. Beat is nice. Disliked the water jet though.
6 introduced alternate armor. It probably has one of the best graphics on the NES. The
credits melody always gives me feels.