Falcon and Blade are notable and there's favorites like Bishop and Spectrum.TBell before Wendy's, but honestly a lot of it comes down to regional preferences. And also the type of fast food joint.
Does Marvel really have a lot of big black heroes? Popularity, not size.
Luke Cage
Black Panther
Storm
Miles
Rhodes
Riri's coming up. Misty and Bishop are there, I guess.
Granted I know you said best and not most popular, but the thought just occurred to me.