CLT and PIT always made US (EAST) the best connecting airline in the nation-as long as you could avoid PHL. PIT still is the best overall (insert standard mournful loss of PIT hub statement here yadda yadda yadda) and even better now as the airport is essentially built for Atlanta sized volume and with it's current moderate volume, never has crowding induced air traffic or ground delays. Also, still some of the best, experienced, helpful employees around.

CLT is equally pleasant in my opinion, but more jammed and more difficult to connect.

The other hub i like is PHX which so far has worked well for me.

Bad hubs? ORD, LAS, IAD, ATL and of course the all time worst: PHL