For me, it's a virtual three-way tie between the d'Deridex-class Warbird, K'Vort-class heavy Bird-of-Prey, and Sovereign-class battleship (although perhaps the K'vort has just the slightest edge over the other two). The first two combine (appropriately) predatory lines with a deadly beauty and power; while the Sovvy somehow conveys an image of sleekness, power, and grace.
Oh, and feel free to include outposts & starbases if you wish. Not that defensive installations are necessarily as exciting, but there is definitely something rather imposing about at least some of them....
