It's unrealistic for BB to get so many followers so quickly, thus I don't like that Oda gave him the title of Yonkou just to ensure that he is one of the biggest threats.
I mean, sure, his crew is undoubtedly among the most powerful crews in the world, just look at the people he gathered, but the Yonkou are a bit more than that. Even if BB's crew is stronger then one Emperor's own crew (not counting those countless smaller crews and divisions with their captains) it's not the status he should get without being in the NW for some years.
Aside from that, I completely agree with you AMA. I'd go with Shanks having the strongest and largest fleet, but BB will have the edge when it comes to crew strength. I always thought BB will be the one to kill Shanks anyway.