If we're talking established, open weight, gi or no-gi, I think it has to look like this:
They're simply leagues ahead of their competition recently.
If we're talking MMA fighters, taken out of the cage and put on a grappling mat, then it gets even easier to decide IMO:
And if we're talking potential prodigies... Kron Gracie, period. 5 pan american championships, 2 world championships, 51 straight submissions from purple to brown belt, and I don't actually think he's lost a match yet. The kicker? The kid is like 21-22 years old.
As for MMA-applicable BJJ... that's a tough call I think. I'm sure Iron Man would be better at deciding that list.
sig credit to Limba