I thought of another one...Well at least maybe. I Love Ultimate Qix on the Genesis and it is based on the arcade game VOLFIED from Japan, (Also known as Qix Neo (on PS1)). I believe it only got an arcade release in Japan...So "unheard of" instead of "underrated" would probably be a better description. Now, thanks mainly to Ultimate Qix on Genesis and the Taito Legends release on PS2, I think some people have heard of it...
Still, I thought it bore mentioning as many people reading this are looking for cool games to play...Arcade or otherwise. FWIW, Ultimate Qix is my Absolute Favorite game on the Genesis! It's one of those rare games, filled with depth, strategy, and fun that I can pop in anytime and come back to again and again. For the record, I don't play to beat it, I play to try for 99.9% on a screen for massive points before moving on...Which is tricky because filling in 80% or more will cause you to move on; So you need to get close but not connect a lot of pieces, then trap the boss in a tiny section, before closing him off...It's so much more fun than it sounds.