I'm going to say go with something extra clean. Track bike or single speed conversion. No lights, no racks and as few cables as possible. Road drops, no track drops or flipped northroads or whatever. I would even take the clips off the pedals.

Don't go with the bike that looks prettiest in daylight—go with the one that will look the best in silhouette. This means also picking one that isn't necessarily set up for riding. Get a fistful of seatpost and no excessively long or short stems. Try to go with something like a 56cm frame—just the right amount of headtube.

Don't try to make it an easter egg hunt for C&V forumites. Go for an icon—something that anyone can look at and say "bike."

So far my favorite is:

but like I mentioned, those pedal clips won't look good in silhouette. And you gotta hide the non-driveside crank arm behind the seat tube!