DEFINETELY SUPER PAPER MARIO! I love this game a lot! And (Don't laugh), I think Poképark 2: Wonders Beyond underrated. I play it because of the battle mechanics and the void blurb that reminds me from the best game of Paper Mario series, but less deep.
Poképark Wii too, I wouldn't consider "underrated" since it is more childish then the sequel (Hey You Pikachu boring flashbacks), but it still has the battle mechanics. It is really great you can win battling by skills and not using Earthquake Blissey or something to win...