AA without a doubt.
At the legacy I work for I've been based all over the world, flown wide bodies into London, flown RNP approaches to a 5,000 foot runway in the pacific islands, done a day turn to Narita, had a 30 hour layover in Tulsa, non-revved to Europe for a two week vacation etc etc etc.
I often think to myself, when I strap in my 737, if I worked at Southwest I'd be saying hello to the rest of my career.
AA hands down if only for the variety. I'd go crazy sitting in a 737 for 30 years.