Agreed on Monster Party, it's pretty cool. Some others (all NES):
Blaster Master (idk if it's "underrated" but it's amazing)
A Boy and His Blob
Captain Comic (a lot of people hate this one, but i dig it; ymmv)
Destiny of an Emperor
Kirby's Adventure (probably not "underrated" either but if you haven't played it, you gotta)
Solstice: The Quest for the Staff of Demnos
Toobin'
Xexyz
Ys
Here's a site that has a review of every (US) NES title, in case there are some you're curious about: http://www.questicle.net/p/list-of-nes-games.html