In GT3, I grew to love the Seattle and Cote d' Azur tracks. I have a 500-hp C5 Corvette in GT4 that just loves these two tracks. The Corvette is slow enough to still be "realistic", but fast enough (with the Sport suspension) to still be completely engaging.
I love the addition of the '87 Buick GNX to GT4, and like that model at most any track. I've got mine wicked up to about 370 hp (almost 500 lb*ft of torque) and I'm still using the stock suspension (not even Sport). I love how that 200-4R transmission lets the speed really wind out in each gear. Top speed about 160-170 at Sarthe II. Don't try that in a stock 1980s GM G-body car!
For the rally races, I really like the Lancia Delta HF Integrale. I haven't spent much time at all suspension-tuning my rally cars, and in stock form, the Lancia's balance agrees best with my driving style. The Peugot's tail jumps out too easily, and some of the Japanese rally cars have too much understeer for my tastes. I like that Lancia at any dirt course, but for some reason, I'm really quick at the Swiss Alps course. It took me a few times to beat the AI car at Grand Canyon "Regular", but I dusted off the AI on the "Hard" version of Swiss Alps the first try. I think it's the wide smooth corners. That Lancia loves to "drift".