Along with Infilitration, Tokido, Justin Wong and ChrisG, SonicFox is one of the best fighting game players at the moment. Though, it is far too soon to say he is one of the best fighting game players of all time. There are players that have won and/or placed in multiple games, throughout multiple genres, for much longer than him.
I find his recent fighting game dominance to be most comparable to ChrisG and Infiltration. Before Chris' recent break from tournaments, he won and/or placed in nearly every game he entered. Next to SonicFox, Infilitration has seen one of the most financially successful tournament runs where he dominated SFIV and SFxT for over a year - winning nearly every tournament he entered (including Evo and Capcom Cup). Next to Daigo, Momochi, Xian and Kazunoko, Infilitration is still one of the best SFIV players and is now one of the best SFV players.
SonicFox is undoubtedly the best NRS player we've seen since REO. He is definitely one of the best fighting game players we've seen in recent history. But, of all time? Way too soon.