If you go on accomplishments it might be Anderson if you go based on who would beat who it's Fedor easily, he would smash Anderson.
If you don't like that line of reasoning then there is a GOAT in each division and no greatest overall.
"All I guarantee is Violence" Wanderlei Silva
HW Fedor, Mir, Cormier
LHW Wanderlei, Shogun, T. Silva
WW Diaz, Swick, Kampmann
LW Gomi, Pettis, Alverez, Khabib N.