Isn't there already a SMS thread to discuss the system? Why clog things up?
Xak I&II are modeled off of Ys I&II, but there is overworld exploration, dungeon crawling, and whatnot. Xak III is more overtly Zelda-like, thanks to its attack-button (versus the previous games' hold-n-bump system a la Ys). There's a fan translation worth seeking out.
Similarly, Efera and Jiliora is a decent, but not extraordinary game in the top-down RPG mold. Like Xak, it's more modeled on Ys (RPG elements like buying armor, levels/stats), but it can be fun if you sort of know what you are doing (look up my thread on the game for a few hints/explanations). There's a reason the game is fairly cheap though, because its quality can be a little suspect. Same with Xak I&II, but those games are so fast-paced and generally a good-enough time that it doesn't stick in your craw as much.
Tower of Druaga is one of the earliest top-down adventure games along with Hydlide, except in arcade format. it too inspired Zelda. The PCE luckily got the best version of the game, an update/remake with more treasures, more accessible game design, and better graphics. Unless you play a translated romhack though, these improvements will be meaningless.
It's an arcade-styled title, not a sprawling adventure - you are limited to figuring out 60 top-down mazes and finding treasures (you get a hint each floor on how to do this). The game is fantastic though; I highly recommend booting it up via Turbo-ED or Ootake or something.
If you want to venture further you can try its sequel, Return of Ishtar; I have a hard time figuring out the game though.