Dragon Quest 3 (and the two previous games) are great on NES. DQ2 is one of the hardest RPGs, though, as the developers admitted they never play tested the final dungeon. First turn AOE insta deaths, mini bosses that respawn, etc. there’s a reason they’re remaking them, though.

Those were inspired by Ultima and Wizardry, though, if you want even older RPGs. Actually, most SSI AD&D games were fantastic, too, and you can access them legally via steam now if you don’t want to futz with DOS, although my favorite Eye of the Beholder games are slightly newer at 1991.