Depends on the rack and where it is.

If it's in a very well visible place, with good security patrols, and the rack is of a good design...any of my bikes.

If it's a poorly designed rack, bad location, or has little to no active security....beater bike.

....on that note, I really should buy a beater sometime....thinking "ladies" chicago schwinn road style bike....weird enough that it might not ever get touched