Several of my favourites
X-Wing, Tie (Fighter and Interceptor), Star Destroyers. Even if the movies are essentially crap, they had very nice spaceships in star wars
USS Enterprise-D (sleek design) and Borg cubes (anti-sleek design) from Star Trek TNG. The D'deridex class romulan warbirds and the Ferengi marauders are pretty awesome as well.
those pyramid ships from Stargate are a pretty nice concept.
My all-time favourites though, are the HUGE, witty, selfconscious spacecraft from Iain M. Banks' culture novels.