Nobody mentioned the best game of all times on any platform yet? Nobody mentioned Hired guns? Thats just jawdropping game for A500! Although it has only 2 kind of enemies per level, everything else is just awesome.
And I would also add two games from Graftgold: Paradroid 90 and Uridium 2. They are just fantastic in terms of programming and overall perfectionism - from open credits to hall of fame.