In my opinion, a "rarity" listing cannot include any factory prototypes or hand-built cars, because these are effectively "one offs" and do not qualify for such a listing since they are "unique", and not really "rare" if someone owns any of them.
A "rarity" listing can only incorporate, genuine PRODUCTION cars available for purchase to the public.
Hence I believe that the term "rare" used for a one-off model is simply silly.
As far as rarities, of course the Scalextric Bug Type 59 is scarce, but if one wants to buy one, he can find one, it is not THAT rare, and enough of them have shown in auctions over the past 10 years alone as to question the "rare" moniker. I personally place them as "scarce".
As an example, the aforementioned Cox Chaparral 2E "mag" INTACT kit is much rarer, even if the built cars are legions.
Rarity also has to go par with "desirability". I personally could not care less if some ugly plastic blob with mickey-mouse motor and chassis produced in the day in the Soviet Empire for consumption by happy little socialist-paradise children is rare or not.
Of all the slot cars produced on the planet in the day and today (not talking about the "serialized, numbered, limited editions" from recent years by dubious manufacturers than ended falling on their face by manipulating their customers a bit too long), and taking in consideration the Scalex Bug, I would say that the "rarest-most-desirable" will be some of the more difficult to obtain and beautiful Japanese, Italian or American kits and RTR's, of which I can give you this brief and certainly not definitive listing, and any of these models MUST of course include their original intact packaging to qualify, as beaters are a dime a dozen out there.
So, in not any particular order, here is sample of what I consider darn tough to obtain at any price, and please note that the first ones MAY have never been issued, and please, don't think about "rare colors" or "rare variation with some cheap bit of plastic added used as clutch" or some other silly implement:
1/ 1/32 Aurora Buick Demolition Demon RTR (if ever sold to the public) None known to exist at this time.
2/ 1/32 Aurora Mercury Demolition Demon RTR (if ever sold to the public) None known to exist at this time.
3/ 1/24 Carrera Ferrari 330P4 (if ever sold to the public) None known to exist at this time.
4/ 1/24 MPC Dodge Ramcharger (if ever sold to the public) None known to exist at this time.
5/ 1/24 Unicar Mercedes-Benz 280SL kit ???
6/ 1/24 Unicar Porsche 906 kit Have car, need kit!
7/ 1/24 International "Furious Fiat" RTR Have ONE car, know of only ONE other MIB.
8/ 1/24 Unique Mako Shark. Only ONE known to exist, MIB.
9/ 1/24 K&B Ford MK2 RTR. Less than 4 MIB known to exist.
10/ 1/24 Classic "Stinger Roadster" kit. Only ONE known to exist MIB!!!
11/ 1/24 Kal-Kar Chaparral 2D. Only TWO known to exist MIB.
12/ 1/24 BZ Chaparral 2E RTR (if issued beyond the 50 or so samples sent all over the planet, ever seen a box?
13/ 1/24 Champion Ford Galaxie kit. Only ONE known to exist MIB.
14/ 1/32 Scalextric Bugatti T59. About two dozens known to exist. I don't mean the "Haviland This" and other replicas, I mean the real thing.
15/ 1/32 Aurora Rover-BRM. Russell Sheldon had one.
16/ 1/24 Unicar Berlina Aerodynamica (ever seen a box?
Etc. I have a listing of over 2500 kits and RTR just from 1957 through 1972, much of it will be in that new book I am finishing now, and thousands of home-racing cars and kits were produced from 1972 to today, of which really NONE can be considered "rare" because all were built for collectors before being built as toys for tots, regardless of their manufacturers main concern and at the very least, HUNDREDS of each are out there. I also must say that any that feature a hand-cast, resin body do not qualify, as I consider them as "one-offs", for any "rarity" listing.