The non-refit Excelsior class.
It personifies Star Trek design in my opinion and has an almost majestic appearence.
I loved that design too - though the first glimpse of it in TSFS was one of the best shots. I recently watched the extras on the DVD to hear the design logic was a Japanese Starship.
More recently the Akira and NX-01 styles have appealed top me. Maybe as it was something reletavly fresh, plus the Sovereign with it's more high-tech look.
The movie era E-Refit style will always be one of the favourites too, though the Nebula and the Intrepid never really appealed to me. They both had some ugly angles - such as the beauty shot of Voyager that was throw around in it's first year. Awful.