The answer depends on whether you want to know about airport operations or the quality of the layover. For airport operations, the smaller the airport, the better! Less traffic, easy in, easy out. No long delays for gates or a takeoff runway.

For the layover, I have to agree with previous posters, SEA and LAS are great entertainment opportunities. On the 737 I liked the long OMA layover, great hotel in the middle of town lots of restaurants (Steak!!!). YVR is also an excellent layover (I know, not domestic...yet...). Our long layover in LAX recently changed to a new location. The hotel is not great but it is an easy walk/bike ride to Venice beach.