Jon Jones has the most impressive resume, and nobody is particularly close.
Jones : Hopefully he can come back to form and absolutely crush DC. I really don't care what he does outside of the octagon because what he does Inside is amazing.If he can win a couple in a row and eventually move to HW that will only cement his greatness.
Anderson Silva: probably the most dangerous striker the UFC has ever seen. Absolutely electric when in his prime.
GSP: the best athlete to ever step foot in the octagon and the best to neutralize the opponents strengths.Big win streak against stiff competition and if he had more of a killer instinct could possibly be considered as the best
HM:
Fédor
Gracie
DJ
Penn
Aldo