If you're a real diver, I don't think it is possible to pick a favorite. By this I simply mean you likely enjoy everywhere you go for it's own reason... so here are all of my favorites - and why:
Australia - First time diving with Minke Whales... an awe inspiring experience
San Salvador Bahamas - 150+ Ft Vis and wall dives with Great Hammerheads
North Carolina - Great Artifact diving and very cool Dives with Sand Tiger Sharks
Roatan Honduras - Best Topside stuff, nice wrecks and walls
Truk Lagoon - The history, the wrecks, the airplanes, tanks etc...
San Diego / Coronados Islands - Fun playing with Sea Lions
Been many other places... all with cool things... but these stick out the most!