Marcelo is a pound for pound grappling GOAT but we have had guys like Jacre, Roger etc all try and succeed at MMA too.
The guy who had the highest ceiling of grapplers turned MMA fighters due to his ability to take people down and have blanketing top game like Khabib was Ricardo Arona. Unfortunately after doing the impossible (beating Wand in pride) his career went wide ways. He never had an offensive point scored on him in ADCC and was a very dominant (albeit boring and safe) grappler. Looked like the Ubereem and Costa kind too. Now he’s only remembered in where is Arona threads, the page slam and the sokudjo KO.