Well, it depends if you want a challenge or just some quick money. I have raced and counted about 95-100 cars that can win the GT500 series. That's about 1/6 of the total number of cars in the game.
For the GT500 series, anything with a power to weight ratio of 4.50 lb/hp or less should be a winner. If you want a challenge, try a few of the GT350 cars, like the Cucso Impreza or the RE Amemiya RX-7. Another goodie is the [R] '96 A-Spec RX-7 (fully modded)...one of my favorite cars in the game. It won't set scorching lap records, but you'll kick some GT-One butt, which is your mission in the GT500/All-Star races.
There are some cars that require a lot of your attention; the Lotus Elise GT1, for example, is frighteningly fast, but a devil of a car to control. The Lister Storm can be a lot of fun, because that tail loves to slide out of every corner. The Venturi LM600 can also be problematic, but a barnstormer, nonetheless.
The GT350 series is a tricky one if you stick to the cars that the AI races. It's open to 591hp, in which anything over 500hp will destroy the competition. But if you use a purpose-built rally car, or a build-it-up-yourself street-going car, you should be on the road to riches in the GT350/500 series. It is too easy to beat the competiton by entering an R30 Skyline Sil.Formula or an S2000 GT1.
I would venture to say the GT350 series is probably the most competitve racing you'll find in GT2, provided you use the right car.