I sometimes look for the longest layovers because it can give me an extra day in another city. A few summers ago I had a 22 hour lay over in Lisbon. It was amazing. I had a nice dinner, slept, had a nice breakfast, walked around the city, and then flew home.
I think the worst are 4 - 5 hour layovers. They are just enough time to leave the airport, but never enough to enjoy the city or relax. I had a 5 hour layover in London a few years ago. It was just enough time for me to take the train into the city, get a cup of tea, freak out, get back on the train, and sit at the airport.