Dragon Slayer II: Xanadu is the precursor of the first Legend of Zelda; it still has several elements typical of full-fledged role-playing games, such as numeric statistics.
Several NES games can be labeled as "Zelda clones", and they all are 100% role-playing games; three of them are closer to Zelda II, but the following four are clearly similar to the first Legend of Zelda:
If you already know the 2nd quest of Zelda 1 by heart and you are looking for more challenge, Faria is a game that has gameplay elements intermediate between the first two Legends of Zelda (it could be considered as "Legend of Zelda 1-and-a-half"). In particular, Faria features five dungeons in the style of Zelda 1, but on multiple floors!
Willow (NES) is a retelling of the Willow movie, with a slightly different plot and new characters.
Crystalis is one of the most acclaimed standalone role-playing games
In its early stages of development, The Legend of Zelda: Oracle of Seasons was supposed to be a remake of Zelda 1. In the final version, it kept the same dungeon bosses and many gameplay elements.
Ultima I: The First Age of Darkness is the start of a different, but equally successful series. The first game features many gameplay elements that are reminiscent of this game. Although Ultima 1 is older by 5 years, the MS-DOS remake is the same age as Zelda 1.
Neutopia and the sequel Neutopia II are games released by Hudson Soft on the TurboGrafx-16 platform. They have similar gameplay with the first Zelda game, however they do have their own features, like magic wands and the compass.