As B_T said, there are so many friggin' great NES/Famicom games, it is hard to put Faxanadu in the TOP 10, but it certainly is one of my faves:
1. Love the atmosphere (it is depressing, lonely, somber)
2. I like the drab aesthetics, since they fit the game (hey, a reason to appreciate browns and ochers!)
3. The music is nice, to boot.
4. REMEMBER YOUR MANTRA*
5. I totally dug the story, the world tree, and how things developed as you progressed in the game
6. The Catholic Church vibe (crusader/paladin, nun, cathedral) adds a bonus point!
* I learned the word "mantra" from this game.