If you just want to focus on the cream-of-the-crop here's what I know:

Contra Arcade- Tall vertical screen, decent detail for the time, with a mechanical sounding soundtrack. The power-ups pop out as real guns, the jumps aren't as high as NES but they're decent, and as said before less levels than the NES version but still good fun.

Super Contra Arcade: I tried it on Xbox Live, I hate that the jumps were so much shorter. Even shorter jumps than the first arcade. I played the demo multiple times I couldn't take it anymore. I'd stick with Super C console versions.

Probotector versions- I'm a silly American so someone else will have to chime in for those. All I know is they put in all robots instead of humans which can be cool in its own right.

Contra- Just go with the Famicom version! The classic Konami code works. It has a special chip so there's lots of awesome animation like waving leaves in the jungle stage and pulsating organs in the final stage, in between levels there's cutscenes with Bill or Lance shooting off their machine guns or talking on their walkie-talkies, there's a final cutscene that would make Arnold himself proud, and even a secret message from the enemy if you put in a code on a second controller. No joke.

Super C- The Famicom version may be the best because I think it gives you 30 lives instead of 10 lives in the code. The code is different by the way.

Contra III- I don't think you can go wrong with any version. If the Probotector story is more interesting let me know.

Contra Hard Corps- Once again I'd go with the Japanese version. In a brutally hard game among other hard games, this version actually gives you 3 hit points. The extra life code which is different ONLY works in this version, and it gives you 70 LIVES!

Operation C- Like Castlevania 2: Belmont's Revenge on Game Boy, it performs shockingly well, has good controls, completely different levels from it's NES counterparts, and an amazing soundtrack for the Game Boy (one of the tracks was remixed in Contra Rebirth, the level with the ostriches). Another Game Boy classic you shouldn't sleep on, a must play.

Thanks for listening, I had to comment on this, THIS is an EVENT! It took them 13 years to finally put console Contra 1 on digital stores!