Greatest of all time would probably mean longest sustained duration of one player dominating over the largest portion of his competition, bar none that is Brett.

Ares/Neo have had a similar level of dominance over the field, but neither of there reigns have been as long as Brett's is.

I haven't been around long for the iClassic era but i'd say Thallen falls into that category now too, as he has sustained long term dominance over a large portion of the competition on iClassic.