Is it because they blatantly cheated, or simply because of PED usage? I'm only asking because if cheating is an issue for you then GSP is definitely off the list because he went out to fight each round slathered in grease for quite a few fights before he was caught.
I assume the poster meant because of PED's. In terms of general cheating, GSP and Anderson both regularly used grease (Google for gifs of Anderson spreading it over his body), Jones used eye pokes. I'm not aware of Fedor doing anything but using the ropes against Lindland, who for his part used the ropes to get Fedor into the corner in the first place.
The point about PED's is that even if everyone uses (quite likely), those who get caught are either so poor they only have access to easily tested PED's (not the case for any MMA fighter at the UFC level, let alone champs), have an IQ hovering around 60 (following cycling instructions is very simple, in fact an IQ of 60 is probably too high a number), or are caught because they're trying to use more than anyone who passes the tests are using (the difference between getting away with driving 10 mph over the speed limit and getting caught because you're going 50 mph over the speed limit).
For my part, I think GSP, Fedor, Anderson and Jones (in no particular order) are the GOATs. Which one is number depends upon when you ask me, a strong case can be made for all four, depending upon the criteria ... though most of the time I guess I go with the "OAT has to be a HW, since weight divisions are artificial" school of thought - most sports simply don't have size categories.
But that too is subjective, I'm I'm old enough to know that expecting everyone to agree is like expecting everyone to agree on what's the best book, the best movie, the best music - ie its never going to happen, people are far too different to agree on subjective things like that.