I used to think I'd always find the original Enterprise the best, and I was quite upset that they changed the design for STTMP. However, the moment we saw the refit Enterprise (or the A, if you prefer), it was love at first sight. Definitely the Enterprise A.
As for other favorite ships, second is the Klingon Battle Cruiser. I've loved the cobra head and bat's wings for many years.
Third is the Defiant. It's so different from all the other Starfleet design, and that's a good thing now and then.