Not sure about underrated, but definitely mostly unknown anymore, would have to be Star Control 2.
The exploration, combat, and humor are all just so involving. I can pick it up now and enjoy it as much as I did decades ago.
Add to it, Marble Drop and Full Tilt from Maxis. Marble Drop was a fun puzzle game with marbles in intricate machines, and Full Tilt was three Pinball games, one of which was the original Space Cadet Windows later used with XP. The other two tables were pirate and dragon slaying based, but equally as engaging for me
I played the hell out of this game back in the day. Still love it.
Extra points for it for the frisbee catching dog bonus game.