Tamiya SRB chassis
When I was a kid in the 80s and 90s the Tamiya cars I lusted after were the Grasshopper, Hornet, Avante, Astute and Egress.  I didn't know anything about the earlier models, simply because I didn't have a catalogue that included them back then.
This chassis is from before my time, coming out in 1979 and including 4 cars: Rough Rider, Sand Scorcher, Ford F150 Ranger XLT and Super Champ.  I was very lucky to find a Ranger in a guy's house I was viewing.  I ended up buying it from him, but not the house!  Although I always keep a look out for old Tamiya cars at boot sales and charity shops, I have never managed to find anything.  This was my first real bit of luck in that respect.
Tamiya re-released the Special Racing Buggy chassis with the re-re Buggy Champ and Scorcher, but haven't brought the F150 Ranger back which I think makes it a bit special.  I get the feeling it was probably the least popular SRB in its time as well, so it might well be the rarest.  Either way, it's a beautiful body, and a great chassis which shows the start of some of the design characteristics that can still be seen in modern releases.  A true classic and a brilliant model.