To contribute some more, Grim Fandango is one of the most underrated games ever. The main reason is it didn't survive last generation because now it is now impossible to get digitally (I still have the original jewel case somewhere, and I have a limited re-release DVD that I bought - what luck!). It has a great story, a huge amount of charm and a great soundtrack. I'd recommend that game to anybody.
Also, I personally find Splinter Cell: Conviction to be massively overrated. You can read my Gamespot review
here if you want to know why.
EDIT: Underrated isn't a great word because it is universally recognized by everyone who's played it as a masterpiece of the P&C adventure genre, but it is still not very well known.