Rally Cars by deAgostini
deAgostini have some great series that make fine collections and they generally get the models made by really good suppliers like Ixo & Universal Hobbies. They are sold new with magazines on a subscription basis but they pretty quickly find their way on to the second-hand collector market. At this stage it becomes a fun project for collectors to track them all down via on-line shops like mine and on eBay. I've handled the Rally Collection a number of times and I'm part way through listing a complete set I bought at auction recently.
They are selling so quickly though that the range on display in the deAgostini collection page of the Little Wheels shop shrinks as fast as I can add more cars.
I suppose it is a function of age that I like the older cars best, the old Saabs & Fords from the 60's & 70's in particular. The second-hand Volvo PV544 which won the 1965 Safari Rally driven by a couple of privateers is a great model with sponsorship stickers from local tractor dealers and the like.
I've still got about 30 more cars to add so keep coming back to see the new additions every day.