I wouldn't say I have a specific favorite but there are a few types that I tend to prefer.
Slight abdominal muscle bulging
http://www.moviesonline.ca/wp-content/uploads/2010/10/milla-jovovich-ultraviolet.jpg
http://images1.fanpop.com/images/photos/2500000/Ultraviolet-Image-ultraviolet-movie-2581055-1400-931.jpg
Plump but not too plump
Any belly with such a round large belly button
A six-pack that isn't over done
Perfectly flat but with minimal to no rib cage visible