Thunder Force = great. I own II, III, IV (it is the U.S. version, but I refuse to call it Lightening Force), all for the Genesis, and V (Perfect System for the U.S. PSX). It's one of the best shmup series ever created, imo.

I may have to pick up a copy of Thunder Spirits at some point. It's the one U.S. released TF game I've never owned.