it depends of experience I mean that you should know the patterns of each game , for me I can beat most easy the first one in the list

Contra 1 nes should be the easiest all around. But the Jpn. version of Hard Corps. would be the one to try for since it gives you a life bar and getting shot doesn't take away your weapons unless it's a killing blow.

There's also the OP slide move that can be abused to avoid any attack. It's so busted that the NA. version could be considered easy despite not having a life bar so that all hits are one-hit kills.