It depends on how you define "best."
If "best" is defined as the hub that caters to the widest possible range of destinations, and the broadest possible connectivity (routes, frequency, capacity, etc.), then I think ATL
is obviously the winner. On sheer depth and breadth, the singular strength and power of what DL
has there is breathtaking. AA
would probably be the "silver medal" in that regard.
If "best" is defined as the hub with the most unique and irreplaceable position, then I think the winner would be MIA
is the singular hub between the U.S. and South and Central America, and such a clear winner that no other hub can ever come close. The dominance of MIA
as the prime U.S. gateway to Latin America is greater than any other U.S. gateway has with any particular respective global region (i.e., JFK
to Europe, SFO
to Asia, etc.).
If "best" is defined as the hub that is most convenient and comfortable to navigate and travel to/from/through, then the answer is infinitely debatable and subjective. We all have our own choices.