Apologies, I drink a lot. I didn't fully illustrate my point. Royce and Renzo have better MMA resumes than Rickson, and clearly rank higher in that sense. Any thoughts on how Rickson does in MMA would have to be based on how he would do head to head, under what rules and would be mere speculation. However, Rickson's legend is sound. His praises are sung by the best of the sport bjj guys, and he did step into the NHB world. Bigfoot, the Loch Ness Monster and Bruce Lee are all hearsay. The myth of Rickson has real roots.
Yes, this is still a question! Tons of MMA fighters have better MMA resumes than Rickson Gracie. Almost every UFC fighter at 185, and above, has a better MMA resume. Hell, they have better resumes below 185. The question was "how good was Rickson Gracie?" I don't see "mma" in that question. That opens this up a bit. Rickson Gracie is better at BJJ than arguably anyone. Rickson is as great at BJJ as anyone will ever be at anything. I would love to see Rickson vs Maia, regardless of the rule set.
I would bet on him against any fighter in the world prior to UFC 1. His brother Royce proved that Gracie JJ had a big advantage over just about every other style, which wouldn't be closed for a few more years. He was better than Royce according to Royce's own testimony. Additionally, Rickson won the two Japanese vale tudo tournaments, and we have surprisingly good documentation that he beat up two other good fighters (by the standards of the day) in no rules quasi streetfights. Plus, he was the Gracie enforcer in a challenge match culture. He was the best MMA fighter in the era before MMA. I doubt he'd be able to compete against the best today, but he did pretty well considering what he had to work with.