Fate at the top of his game is a fundamental force of the universe as A Lord of Order... However on the reverse of that assuming by top of their game you mean Scott is going to go for a total annihilation via dumping all power from starheart.... Scott would win if he didn't care about taking out a good portion of the universe doing it. Also which Dr. Fate?
Alan at his best is what, High Trans? Maybe around Post Crisis Darkseid level or a little higher?
At his best Fate was performing Universal-level spells.
IIRC he controlled Darkseid, Highfather, Orion, and Etrigan at the same time and channeled their combined powers, and after that was over Darkseid got pissed that he had been used but backed off from a fight because he knew Fate would have won.
If Darkseid's pride takes a back seat to his better judgment, then I'd say that Classic Fate was much more powerful than him.