First, last and always my Defiant class. I've even skipped getting a ship at Rear Admiral level just to go from my refit to my retrofit. And now I am awaiting our Starbase to reach tier 3 for my fleet Defiant.
I've tried lots of different ships, and the Heavy Escort Cruiser from the c-store is another favorite, but I always return to the U.S.S. Avenger B (eventually to be the U.S.S. Avenger C).