Oh my, another thread of revisionist history and opinions!
Without modern record keeping we do not know who Rickson fought outside his official late career MMA stats, and the few publicized dojo fights (Anjo, Duarte x2, Zulu x2), with the latter obviously just a sample of who he fought. We know he did not turn down fights if he was challenged, and had a significant part in the Luta Livre wars. The late career he dominated against relatively less competition, though always impressive against multiple opponents a night, no scouting report, and all but one outweighing him by 20 to 100 pounds all finishes. No one does that if they were protecting a legacy. Funaki outweighed him by 20 ponds and was 11 years his junior.
So a few benchmarks must be used to measure by
1. Rickson always beats Royce, so whoever Royce beat, Rickson would beat.
Royce would have beaten Saku if it were a standard match (Rewatch the fight its true) but it went for 90 minutes and I won't spoil the reason for the outcome. Royce beat him in the rematch. If Royce does this, Rickson does it 10x better. So Saku looses to Rickson.
Royce beat Dan Severn (one of my favorite fights back in the day) Rickson beats him easily.
Royce beat Ken Shamrock, Ken Shamrock beat Bas Rutten, so Rickson is 10x better than Royce.
2. Look at his skills vs the opponents mentioned.
It has been said a good MMA fighter is a brown belt in everything. Rickson was/is Blackbelt level in obviously GJJ (not sport BJJ which is not always a good transition to MMA/NHB), but also Judo, Sambo, and amateur wrestling. He could also punch.
Mark Coleman was a killer but he was susceptible to someone who could catch him in a transition to the ground. Rickson's a master in the transition, and unparalleled on his back. Either a sub or a sweep, which Coleman would not survive with Rickson on top.
Henderson also had a lot of trouble with BJJ fighters in that time period. Henderson is not KO'ing Rickson and can't just out wrestle him. The more he tries the more he opens himself up.
Oleg - Rickson wrestles him and subs from top. Renzo beat Oleg, Rickson beats Renzo.
Tito - as good as Tito was, don't see him taking Rickson down. Standing Guillotine.
Don Frye - some say it would go like it did with Amaury Bitetti, but Frye won because Betetti could not take him down. Rickson's takedowns are much more dynamic and varied, it would have been unlikely if Frye would not have ended up on his back at some point. Rickson Gn'P from mount.
Frank Shamrock - would be a good fight, but Frank's strengths are also Rickson's strengths and Rickson was stronger.
Igor - Man Igor was solid! Low center of gravity, lightning quick thunderous hands. Outweighed Rickson by 20 pounds. Would have been good.
Of course, no gloves or hand wrappings, with no time limits I don't see Rickson in his prime losing to anybody. Its like he was designed to succeed in that form of combat.