They both are fighters who pulled off extraordinary feats, hard to say who was more impressive. I'm biased and Randy was one of my favorite fighters, he was old, the underdog, used smart gameplans wrestling and cardio to beat guys he had no business fighting. Randy's clinchwork is legendary. Hendo carried power through 3 weight classes and the later part of his career. Both pound for pound guys.
I feel like Hendo's skillset narrowed to just his power, while Randy's expanded as he got older to bjj defense, head movement, etc. It's a really difficult choice because Hendo had some of the better wins later on, but my vote goes to Randy.