I didn't say that, but since you want to get into a tiff on the subject, Circle of the Moon wasn't underrated. In fact, it's considered by most to be the best GBA Castlevania.
But seriously, here are some of my underrated games, and I'll tell you a little about why they're underrated:
Wild Arms: Alter Code: F (PS2) -
This game was actually panned by most critics as being stale and not very well done graphically. I was a huge fan of the original game, (the rest in the series sucked pretty bad,) and to see the magnificent story and soundtrack fully realized in 3d was wonderful. It's one of the few console RPGs that I can actually play the whole way through without getting bored.
Dark Law: The Meaning Of Death (SNES) -
While it was only released in Japan, Gideon Zhi of AGTP put out a translation of it a while back. It's technically an RPG, but it has a lot of interactive adventure aspects, pushing it up to the next level of entertainment. A lot of people don't like it, but I find it strangely addictive.
Fusion Frenzy (XBox) -
Critics hate this game. The whole mini/party game genre is usually looked down upon by them, and most of it is justified. I have some great memories of playing this game with my old band in hotel rooms though, and sometimes I wish I could get everyone together to play it again. I never played the sequel, but I've seen some horrible reviews for it as well.
Extermination (PS2) -
Before the days of Metal Gear Solid 2 and other 3rd person formulaic shooters, there was a game called Extermination, where the hero is roaming the halls of an abandoned military base in a violent snowstorm. It was kind of rigid storywise, and these days I don't even think someone would develop or produce this game, but it was pretty sick when it first came out, all those years ago. I think it was the first PS2 game I bought, actually.
Hoshigami: Ruining Blue Earth (PSX) -
Hoshigami is a tactical strategy game, in the vein of FF Tactics and Disgaea. This game was excellent for its time, utilizing such features as chained attacks, and the ability to literally bounce an enemy back and forth on the battlefield with a succession of attacks. It was very heady stuff at the time, but NIS games came along and rendered it irrelevant.
Solar Jetman (NES) -
Ahh, Solar Jetman. The tales of a space-age treasure hunter trying to amass wealth on a variety of planets. Complete with independent ship and jetpack controls, planetary gravity, and insane difficulty.
Jackal (NES) -
Konami. Jeeps. Four-Way Rockets. Power Metal Soundtrack. I need say no more.