Fireball XL5 – I’ve got a really nice diecast model of it, the decent capsule/cabin detatches and is held in place by a magnet. It’s about a foot long.
Babylon 5 Shadow ships, amazing looking ships. Always wanted a model, but we’re only available as resin garage kits, and I never got around to tracking one down.
As others have said, Culture ships are immensely appealing, but until the book of Banks’ drawings and concepts comes out, difficult to know what they really look like.
Also difficult to pin down which is a favourite, really, but ‘Grey Area’ might come near the top of the list.
Also, Blake’s 7 ‘Liberator’.