Hmm, let me describe how I feel:
US: Lots of muscle, not too good handling, sometimes interesting design. I always love it when an old classic is brought back in a truly spectacular way. The new Mustang was a success obviously, as well as a few concepts by Chevy and Dodge. The Chrysler Crossfire is stunningly designed as well.
Japan: Every car has something interesting in it. BE it a design feature, or just a simple rim design. (Nissan cube anyone?) K cars aren't that bad, not fast but I find them to be interesting. Also, a lot of the Japanese cars have very good handling, even some of the FFs.
Italy: One of my favorites. I've always loved Fiat, as well as Alfa Romeo. Like the US, this country always surprises me with a new model that is based on old ones. Man that new 500 is cool looking, and how 'bout that new 8C?
Germany: BMWs are good, particularly the classics, but I'm not a huge fan of the newer ones. the only ones I like are M5, and M3, not much else. The Z4 looks OK, but I'd prefer a 2002 Turbo. Audi has had some good concepts lately, many of which are in the game. Who can resist that R8? How about that Pikes Peak car? All amazing, if I do say so myself. VW has taken some risks lately. THe NArdo is truly amazing in comparison to other cars, and I actually love that color. I just wish some of these cars were more accurately portrayed in game.
France: Renault is by far my favorite here, with it's admitedly odd styling, I think they managed to produce an image that many will recognize immediately. Peugeot is good too, as I find they focus a lot on one thing, and no matter how much people hate it, they keep working on it until it's loved by everyone. Citroen only really appeals to me with the Xsara and 2CV, although it's fun to race a few "1 lap magic" races in the snow with some of the family cars.
UK: Lotus is by far the winner in this category. I've always envied the way they focus on light weight and sporty cars, unlike a lot of the competition. The Elan, Europa, and Esprit are all amazingly well designed (besides the obvious quality slippage they've had in the past *cough*Lotus Elise?*cough*) Apart from they're fascinating approach, I find that a lot of Lotus designs appear to be inspired by animals. TVR is another good one, but often too powerful for their own good. If they had just used the Lotus approach... I've also had this weird fascination with Jenson Interceptors. Don't know why really, but they have a very interesting shape. Jaguars are the other main brand that stands out for me. Who can resist an E-Type, or XJ220?
Australia: I don't know much about them, but I do know that they're often very well handling, and look cool as well. Is it just me, or do most of them relate to Ford in some way? Not to mention the Pontiac GTO that's really Australian.
Other: A few countries don't have many cars, so I'll just merge 'em. Unlike many people, I find Volvo's style to be inspirational in many ways. The fact that they always keep safety in mind, and learn from all the mistakes they made is a very good approach for anyone to take. Although boxy in the past, it's obvious Volvo knows how to take very interesting shapes and mold them into their cars. Look at some of the taillights on newer concepts for an example. Seat is an interesting brand. It's another one I don't know much about, but I do know that I've seen it compared with famous hot hatches, and it wasn't too bad. I like the strange design features they use, and love that bold yellow color. The Spyker looks... interesting... from the inside, but luckily I don't need to look at that while racing in GT4. The outside however, is very round, and reminds me of the first Corvettes, with that shiny trim.
I think I missed a few countries, but these are the ones that come to mind.