Correct answer to the question is:
Stealth cars.
After: limited chrome line cars
After: zone limited cars (Gamestop CoT, Edge, XJ13...)
These cars are impossibile to find in EVERY collection.
Then maybe the 370Z prize car, and the GT Academy Nissans.
Then we have to look at UCD/OCD rarity.
UCD TOP 3 are: FX45, Xanavi Hiroto GTR and Takata Dome NSX '03
According to TornadoAlley thread lists (more than 2500 game days covered):
# - manifacturer - car - nr. of appearances in UCD
1st Subaru Impreza Rally Car #7 '03 1
2nd Cadillac Cien Concept '02 2
2nd Audi Audi R8 LMS Race Car '09 2
4th Alfa Romeo 155 2.5 V6 TI #8 '93 3
4th Acura DN-X Concept '02 3
4th Gran Turismo Formula Gran Turismo 3
4th Honda Takata Dome NSX #18 '03 3
4th Infiniti FX45 Concept '02 3
4th Nissan Xanavi Hiroto GT-R #22 '01 3
4th Peugeot 205 Turbo 16 Evolution 2 Rally Car '86 3
10th Ford GT '02 4
10th i concept 4
10th ... others
The Subaru and the Peugeot are in OCD. The Cien, the DNX and the R8 are also prize cars. The Alfa was in OCD some months ago. The FGT is in OCD every week.
I was lucky to find the FX45 and the Xanavi Hiroto GTR.
The Takata Dome NSX '03 is the only one I haven't. Quite impossible to find, best candidate to be my 1147th car model...
More.
According to mygt garages (more than 20000 users), we have a confirm that GT5 rarity is also user garage rarity, aside of the fact that duping generally destroyed some stats (the NSX Takata Dome is quite popular.... not speaking of the X2010).
Rare cars also rare in garages:
- Stealths
- FX45
- GTR Xanavi Hiroto
- Ford GT '02
- Mitsubishi i-concept
Also rare in UCD:
- CZ3 Tarmac '01 (only six)
Not popular as there are too many versions: NSX, S2000, Tuscani, Lexus GS300, MX5, RX7, RX8, GTO/3000GT, Silvia, Skyline, Z, Aristo, Legacy, Prelude, FTO
Not popular because there is another DM version:
- March 12c (Micra), Yaris F (Vitz), ist (Scion), Toyota Voltz (Vibe), Vauxhall Vectra (Opel), MRS V (MR2), Mazda 6 concept '01 (Atenza)
Strange things: can you give an answer?????
- Ballade CRX, G35 '03, all Primera, Pontiac Sunfire, Suzuki S2, Celica ST162
- LEXUS ENEOS '08 (Premium!!!!!!)
- Atenza '05 (why the best one is the less chosen?????)
- Mirage 1400 (too ugly???)
- GTR Loctite Zexel '00 (why only this?)
Another interesting key would be:
what is the largest gap for 2 succeeding appearances?
I did a search between UCD rarest (last seen Impreza WRC '03 was 14000 days ago!), aside of the OCD buyable, and found this:
1st CZ3 Tarmac '01 about 11000 days (last low level car bought, for me, last week)
2nd FX45 more than 10000 days
3rd Xanavi Hiroto about 9800 days
4th Takata Dome NSX #18 '03 about 6900 days
Now you know how much you can ask for trade
PS Sorry for long post, but question was for it