I have never been a fan of the 'lay & pray' wrestling style fighters, I dont mind high level wrestlers/grapplers that actually go to work when they get to the ground, looking for a submission or ground n pound but this laying on top for control for the entire round.. nah not for me.
I much prefer to watch high level 'technical' strikers whom also have a good ground game. So for me a prime Anderson Silva would be my pick for GOAT, however imo he has carried on fighting too long and in recent times has been a shadow of his former self. I also think in his prime GSP should be a good contender for one the best of all time.
Out of the modern fighters, Jon Jones would be up there but for me the failed drugs tests etc put his success in some doubt.