Maia? Roger Gracie? Fucking Rickson? Get real. There is naught, nor ought there be, no one so exalted on the face of god's green Earth as that king of grappling, Milo of Croton.
Croton was a god damn hornets nest of pissed off nude wrestlers (think Dagestan, but older and more nude), and this bad ass was the best motherfucker there. He practically invented bro-science, taking progressive resistance to insane levels, lifting a calf until it became a cow, and beyond.
In terms of grappling, he won six men's wrestling titles between 536 and 520 BCE. Compare that to Karelin, who only won 3. Milo also won seven crowns at the Pythian Games, ten at the Isthmian Games, and nine at the Nemean Games. Milo was a five-time Periodonikes, a "grand slam" sort of title bestowed on the winner of all four festivals in the same cycle. Milo's career at the highest level of competition must have spanned 24 years.
He's also indirectly responsible for ruining everyone's life via math, when he saved Pythagoras's life. A pillar collapsed in a banquet hall and he supported the roof until Pythagoras could reach safety. Pythagoras would later go on to create an annoying ass theorem that you probably forgot about after highschool.
I know, I know, "but what about MMA?" Spare me. Let me kick it up a notch, Milo didn't go into some nambly pambly MMA promotion, he fought in a god damn war. He dressed up as Hercules, patron saint of wrestling, replete with Olympic crowns, club, a bow and some arrows, a lion skin, and a prominently displayed tiny dong, and led the charge, turning back the first wave of troops. Show me any other grappler that succeeded in war with his grappling skill.
Honorable mentions to:
Arrhichion, who was such a ride or die grappler that never tapped.
Polydamas, who fucking out grappled a lion AND a bull, and took on 3 dudes at once.
Dioxippus, who double under-ed a dude who was throwing god damn javelins at him, threw him down, took away his sword, and stepped on his neck (Dioxippus later killing himself by falling on his own sword, because he was the only bad enough motherfucker that could kill him).