musha is great on gigadrive (terrible name I know, but who cares, so much fun)
super aleste/space megaforce on snes (still a little slow but very fun)
same developers BTW
if they ever put out an update for GPEngine than hopefully you'll be able to run blazing lazers, another great game. i guess there's a few games similar to that also on PCE that work well with that emu (final soldier? i forget)
also on genesis you got raiden, vapor trail, grind stormer, phelios, truxton (hard one!!).. for horizontal try hellfire, wings of wor, steel empire, zero wing, insector-x, thunderforce series (doesn't work yet on GP32) and elemental master (also doesnt work yet) -- gley lancer is cool, gaiares is a great one (don't think it works?) -- whip rush is fun, arrow flash not so much. TONS of shooters for genesis! aero blasters.. uh.. curse..