I don't understand how Contra III is beating Contra (NES) and Super C (NES). I was torn between the two NES titles, but ultimately went with Contra (NES), because it was an improvement over the arcade (sorry! the sprites were out of proportion to the play area in the arcade).
As for Contra III, you guys aren't forgetting about the dull overhead scenes, are you? Because they really drag the game down a notch. OH SNAP! Super C also had overhead areas... THAT'S WHY CONTRA (NES) kicks all sorts of ass

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Contra Hard Corps is nice, but it's also the goofiest Contra out there (and that's saying something!).
The only Contra I haven't played is the GBA one, and that's because I really never played any of my GBA games. I actually forgot that I own GBA. I need to get one of those updated, illuminated screen Gameboys....