Randy shouldn't retire. He's old and all but he's still winning so there's really no need, granted his fight against Coleman was an obvious win though. I'll make my decision based on his next fight against a legitimate fighter, no offense to Coleman.
Cro Cop should retire. As much as I don't want to him to, he's just not the same. His fight against Perosh showed me that. Sure he won but he never really came close to stopping it nor did his striking look impressive. The fighter he was facing wasn't even UFC quality and he couldn't even put him away, a cut doesn't count. The man is a legend but he'll eventually just get horribly injured if he keeps fighting.
As for Big Nog, all he needs to do is stop eating punches and learn to block/evade. His defense coach isn't teaching him anything, unless it's to block punches with his face. It's not like he has an iron chin or something, the guy gets rocked in all of his fights. All he needs to do is work on his striking, both offensively and defensively, and he'll be fine. He shouldn't retire yet.