Currently its Fedor but Jones is literally on his doorstep.
I am finding it more and more difficult to compare the modern Unified game to the fighting that Fedor did with the more dangerous rules of PrideFC though.
Its almost like comparing Formula 1 to Indycar drivers, the rules/judging/environment make a huge impact.
Jones' strength of resume is insane, but unless you actively were a part of Fedor's reign you can't accurately understand it. Even many of guys who now people see as 'cans' were interesting tests. Fedor defeated a lot of guys at their strengths just the way Jones has done and transitioned better than almost everyone I've seen.
Fedor still remains the most 'undefeatable' fighter I've seen at his very best. There are a few small holes in Jones's armour and has been matched at times by others, losing the odd round, where as Fedor was head and shoulders about everyone for many years until people caught up.
Jones is basically the equivalent of Fedor for the Unified Rules/cage as Fedor was for Pride in the ring.
A win or two more or if he takes his streak right through 2015, he cannot be denied as outright GOAT, but currently:
1. Fedor
2. Jones
3. Anderson
4. GSP
5. Aldo