Honestly for me it come down to Fedor or GSP.
Young GSP was a killer who was looking to finish you. After the Serra fight iirc, GSP's style became more like scoring and point fighting. But more than that perhaps his ability to control his opponent and be at the least risk for damage was GOAT level in that respect. In that way, GSP's argument for p4p GOAT is very much like that of Floyd Mayweather's to boxing. Takes the least amount of damage (for the most part, we all remember the Hendricks fight of course) and controls the opponent so effectively to the point where they never pose a threat to him no matter how good they are. That takes an incredible amount of skill even though GSP at whatever point started "playing the game" more than going in there looking to put you away first.
Other people can attest to or word it better than this but Fedor in his prime...he was the Terminator *everywhere*. Pride had the best HW division in the sport in Fedor's heyday and he not only went undefeated in it and came to *kill* you every fight but a lot of the time he would beat his opponent at their own game (off the top of my head submitting some of the best grapplers of his time in Mark Coleman and Kevin Randleman, ultimately outstriking prime Mirko Cro Cop in a dang ring in 2005, etc...a great many times he didn't play to his strengths in a fight, he played to YOUR strengths and showed you that he's better at them than you). No matter who it is, where the fight went, what the physical advantages were, you were always in an immense amount of danger of being son'd against Fedor. In his prime he always found a way to win, he always found a way to get his opponent out of there or to dominate them in the pursuit of a finish. He was a killer if there ever was one in the history of the sport despite him being so quiet and mild mannered.
All time and p4p I have to give the edge to Fedor Emelianenko in deserving that number one spot because not only did he have that insane undefeated streak but beat if not finished the best of his time in the HW division, and even post-Pride and post-prime if you will he still had some really good wins finishing Arlovski and Tim Sylvia, the previous two pioneers of the UFC's HW division while Fedor was tearing it up in Pride's.
Anderson Silva in his prime is obviously in the discussion too but I do think Fedor and GSP retrospectively pulled away from him a little in the GOAT discussion.