The rarest car has to be the Hotknife. It's never driven by anyone and is only seen after getting all Golds at the Driving school, and that's not an easy thing to do.
wrong, it is the phoenix as you only drive it in a race and you can only obtain it by failing the race
I have to disagree! The Phoenix shows up in the race yes but it's also seen during the 'Home in the Hills' mission when you chase after Big Poppa (I think that's the Vagos guy's name) driving one. So that's twice you get to see that car without doing anything special other than just doing missions. In the case of the Hotknife you ONLY see it after getting Golds at the school! In my books, this makes it much rarer.
Plus there's also the fact that the Phoenix is supposed to show up on the streets of the game but due to a bug of some sort, it doesn't. Just check the 'cargrp.dat' file and you'll see it listed. You won't find the Hotknife there in the normal spots, it's only listed under the 'fast cars' and 'funhouse' cheats.
Actually, the Hotknife is found on the streets, but I've only seen it during Vigilante missions.