Thats a hard question to answer. First of all, its not T nelson downs. He was certainly one of the greatest for everything he did but he couldn't be the best because he was standing on the ground while the rest of us get to stand on his shoulders.
If i'm making a list based on skill, and not (necessarily) entertainment value. The guys who can do crazy stuff that others can't do and the guys who do it the smoothest. The list would go something like this:
Giacomo Bertini
Reed McClintock
Kurtis Kam
Kainoa Harbottle
Rune Klan
Eric Jones
Gary Kurtz
Troy Hoosier
Ponta the Smith
Hmm depends on which genres I'd judge them on.
To me, I would say the best coin magicians are Giacomo Bertini, Kainoa Harbottle and David Roth since I love their techniques and they're the ones who inspired me the most.
But on the other hand, if I'd judge on performance, excluding the ones I've already mentioned, I'd probably add Eric Jones and Michael Rubinstein.
But if I'd stop being an over-analytic ass and get to the point I'd just say Giacomo Bertini.