I had the Genesis version and I love it and have a lot of nostalgia for it. Love the unique character select screen music more than the original, and who could forget the FERGALITY!.
Buuuut, yeah I also remember renting the SNES version because it just was better.
Also, I got MK II for the Game Boy... I was a bit of a fanatic when I was a kid so I needed a portable version. It was very downscaled of course and had less characters and stages, BUT, I lucked out because at least it plays decently, in fact it's the only good original Game Boy MK game, as MK 1 or 3 on GB suck ass, I came to learn much later, dodged those 2.