I don't really like mega man 11 but it's probably fine for a first entry even though it has mm7/8 style graphics and the dual gear system. (Which basically lets you speed yourself up or power up your weapons on a limited basis)
My personal favorite is mega man 9, that game is just so clean, with killer weapons, stages, bosses and music. I've played it so many times and done buster only, which was a real pain on final wily.
Mm11 is similarly also good, the weapons are purposefully a bit harder to use (except the triple blades lol) but it's still a good experience. I trust almost anything this developer does, given that they were able to make sequels this faithful that long after the fact and iirc they were also responsible for the great Castlevania 3 reimaginings in Bloodstained Curse of the Moon 1 and 2.
Out of the proper NES titles, they've all got their own issues, but it's probably 2 > 4 > 1 > 3 > 6 > 5. I also have fond memories of 4, and was happy to buster only that one as well.