USS Enterprise (Star Trek)
Millenium Falcon (Star Wars)
Star Destroyer (Star Wars)
Runnerups might be the Klingon D7/K'Tinga or Bird of Prey, because they appeared fairly often in the Star Trek movies and series and have fairly recognizable shapes. Though the Enterprise D would probably still beat them. I guess the Borg Cube could qualify there, too, not many sci-fi franchises use that shape and the Borg themselves are also fairly iconic.
Stargate's Goa'uld yramid Ships might qualify, though I am not sure I know the class names, at least not anymore. But if you show me a space ship in shape of a pyramid, I'll think Stargate first.
Oh, and I think the TARDIS is a pretty decent pick, but I think Star Wars will win out. Triangles and Falcon are just too iconic. Not many would recognize it as British police box perhaps (or know that's not a phone booth), but if you literally created an icon or schematic display for it, people would more likely say TARDIS than anything else (other than perhaps "No clue". Just not as many as people that would recognize a Star Destroyer, Falcon or Enterprise shape...)
If we were talking spacefighters, X-Wing and TIE Fighter would probably the top picks. And I don't really know what would be my third. Babylon 5's Star Fury perhaps, but B5 might be comparatively obscure. Maybe that ship from Last Starfighter? Not entirely certain I'd always recognize it, I haven't even watched that movie, just heard about it, and it's a really old movie, not sure many people still recognize it.