Underrated:
Jaws (NES) - Fun in an Atari 2600 kind of way
Deadly Towers (NES) - Super challenging at first, but if you play it carefully, it actually gets pretty easy (and fun) about halfway through
E.T. (2600) - OK, it has its flaws, but people treat it like it murdered their mother or something, and it just ain't that bad. It's actually decent if you have the skill to avoid falling in the pits.
Super Mario Bros. 2 (NES) - Many people like it, but many don't because it didn't start out as a true Mario game and the gameplay reflects that. But I love it!
Plus, the entire Odyssey2 library.
Overrated:
Populous (PC) - Complete snoozefest for me.
I haven't played it enough to be sure, I'm kinda thinking Katamari Damacy (PS2) is overrated. Charming, but it didn't "grab" me, at least not yet. Will I be excommunicated from DP for this?
I'm having a similar problem with Kingdom Hearts (PS2). Walk around, mash buttons to kill monsters, watch long cutscene, repeat. Seems kinda tedious. Cool concept though. I really want to see KH2 for the Tron segments, but if the gameplay hasn't been improved, I'll have a hard time bringing myself to spend the money.