Odd list.
Greatest fighters should be the best guys who were dominant for long stretches of time and had a huge impact on the sport of MMA.
As such guys like Matt Hughes, Tito Ortiz, Jose Aldo, etc should be considered as well.
The whole steroids aspect obviously makes it a question of best "clean fighters" in which case, Silva, (both of them how ironic) and Jones should not be on the list despite their huge contributions to the sport of MMA.
Also Mighty Mouse? fighting a bunch of B level 125 pounders seems like an odd endorsement for a top ten list.
My list in no particular order:
Fedor
GSP
Couture
Ortiz
Aldo
Cruz
McGregor/Rousey
Hendo
Penn
Hughes