Flat protection, extended milage and ride quality are all important to me and its tough to get all that in one package. Up to this point Schwalbe Marathon Supremes have given the best experience to date. They stick like glue on paved roads, are extremely light and they're in their third flat free year. Pro-rated based on the time I spend on different bikes - they've gone over 6,000 kms and are good for another 6,000.
I'd expect less milage if used for loaded touring, and thats part of what they'll be doing later this year. In preparation for that I've gone to the new Michlin Protek Max tubes. They're a little heavier, but retain air pressure better as well as being self healing.
Everyone seems to have their own preferences - thats what works for me.