Assuming we aren't taking PEDs into consideration at all:
1. Jon Jones
2. GSP
3. Anderson Silva
4. Jose Aldo
5. Fedor
So the 5th is Fedor on my list, could be 4th too but I have him at 5. Then in consideration: DC, Penn, Khabib, McGregor, Cruz, Mighty Mouse. Then a tier down would be...I guess Cejudo, Stipe, Nunes, Schevchenko (if women even count that is).
In my humble opinion Demetrious Johnson is always overrated. He was great, but his competition was pretty abysmal overall and he got a fake win over Ian McCall then lost to Cejudo + lost to Cruz and Pickett in the past. I could excuse the losses but he never faced any "good" besides Dodson, Joe B. and Cejudo. But he's great no doubt, just not going to put him above Aldo or Fedor, and I wouldn't put him above DC right now either. Something to be said to win in two divisions, Mighty Mouse never did that + faced mediocre fighters a lot.