I have to give another vote for the refit Constitution as portrayed in TMP. Just an absolute work of art. It still looked good in TMP-TUC, of course, but the work of ILM in the subsequent films, as good as their people are, just never captured the beauty of the ship like the way Douglas Trumbull and his team filmed it for TMP. He really had the care for the product and an attention to detail that makes the visual effects of TMP stand up to this day. And beauty of the effects shots aside, I just like that design, inside and out. It really looks practical and functional as well as beautiful.
Beyond that, I do like the Galaxy and the Intrepid. I really am much more fond of the Intrepid than most people seem to be here. I also really like the Defiant. Really believable as a Federation warship.
The Excelsior has never particularly grown on me. Nor have most of the newer designs like the Soverign, the Akira, the NX-01, and so forth.