The majority of my favorite cars European, especially the exotics such as Lotus, Pagani, RUF etc. I also like the classic European vehicles a lot, like the old Merc. Gullwing, Jag. e-type, Alpine 1600s, etc. In general I find the European vehicles handle well but are also full of eccentric little quirks that make them a blast to drive.
The American cars mostly drive too much like our cars really drive. Fine in a straight line. Often they have lots of power, but not exactly stellar in the handling department, with only a couple exceptions (such as the newer Corvettes, which handle beautifully). Kind of fun on occasion, but that's about it for me. (Most Americans, BTW, consider a sports car to be a car that goes fast in a straight line. This is really a muscle car, but the vast majority of my fellow americans I talk to just don't see the distinction between a true sports car and "a box with a big engine".)
My least favorite by far are the Japanese cars. Note that I currently own 2 Toyotas (2 models of the Prius), so I'm perfectly happy with them in Real Life, but find that the Japanese cars in GT4 seem to lack character. Also, I get the strangest idea driving them that they really are all just a little bit "too good". They seem to suffer from less tire-spin on acceleration, have pretty universally excellent brakes, wear their tires less rapidly (the Tommy is especially offensive in this regard, making R3s last as long R1s on many of my cars from other origins with similar power), and in general lack the handling "quirks" which might make them more fun to drive. For example I love just about all the MR cars in the game, but find the NSX and MRII to be an absolute bore to drive. Also, way too many of them are "ugly boxes" and boy to I hate ugly boxes! I can drive ugly boxes in real life whenever I want, but buddy, give me a Lotus any day if I don't have to pay real money for it! :-)))
(The Germans also make way too many ugly boxes!)