While I love pretty much every vector game out there I'd have to pick either Battlezone or Solar Quest. I don't think I could choose between them! Solar Quest, while simple in game-play terms, has a purity and smoothness that I just love. There's just something really satisfying flying that wee ship around the screen - you can pull off some really slick moves...
Battlezone. Well, where do you start? Amazing atmosphere, a great sense of military desolation (to coin a phrase) and great controls. An obvious choice, perhaps, but it is a true classic. OT: Has anyone played Vector Tanks or the sequel on the iPhone? A stunning game by Peter Hirschberg.