Oh, I love it when we talk aesthetics. So much that I'll give ya one from each game... I probably can't single out just one ship anyway.
WC1: Sivar (Honestly, what is up with that cauldron-looking thing?)
WC2: Kilrathi supply depot
WC3: Can I say "half the damn game" and leave it at that? Seriously, though: Kilrathi light destroyer.
WC4: Does the Confed shuttle count? If not, Longbow.
WCP: Seahawk
SO: Lamprey
Armada: Banshee (Propellers? Give me a break)
Priv: Drayman. Not that there's anything wrong with it, but it feels kinda uninspired to me, while all the other designs in the game are superb.
Priv2: Toss up between Tacon and Aurora.