X-com.
Alright, maybe calling that underrated is a bit of a stretch. X-com Apocalypse was pretty damn good, too, but many people don't like it due to the constraints it puts on you (cityscape vs worldscape, alien weaponry comes in stages rather than giving you the near top tier guns in the beginning, etc.). I'd say, if you haven't played that one yet go get a copy from Y2K games - they're selling it again, and it's awesome.
I know I'm going to get some neigh saying, but I'm actually a bit excited over the new X-com being made, as well. Unfortunately, I'll probably be forced to watch it go on the shelves without a platform to play it on .
Did I mention X-com?