This is a difficult question which varies based on your circumstances. In general, one terminal airports are easier to transfer. Airports with more than one terminal but with the buildings connected or with fast trains connecting them aren't bad.
Some airports are served by one large airline so if there is a problem, there are no other airlines to pick up the slack. US Airways is a fine airline (reasonably ok) and CLT is an easy to transfer airport. However, if your flight is cancelled, there might not be another airline going to the same place.
In my opinion, Singapore is a nice place to transfer. In the US, CLT isn't bad but note the caveat above. SFO and SEA are nice, too. DCA is nice only because you can visit a museum if you have time to spare....just in, rush to see a few things, then back to the airport. Glasgow is a bit spartan but I've connected there before....ditto for Malaga.