Galaga Legions and Galaga Legions DX. Both games are incredibly cool modernized takes on the formation and capture mechanics of Galaga as well as being excellent games in their own right, both were heavily overshadowed by the respective releases of Pac-Man Championship Edition and Pac-Man Championship Edition DX, and neither is on PC.
Mushihimesama Futari. The first game is on PC, but the massively superior sequel isn’t.
Ginga Force. Okay, this one’s a little selfish; despite how much I loved the developer’s previous work, Eschatos, I never picked up the follow-up for whatever reason, and a PC release would let me do that and would almost certainly cost less than importing a copy on Xbox 360.
M2’s various ShotTriggers releases, including Battle Garegga and Dangun Feveron. Arcade-perfect releases with loads of bonus features, but they’re locked to consoles.