It depends entirely upon your preferences, really. The point of rFactor is that it's a modding kit, so there's a variety of cars available in all shapes & sizes. Do you like open wheelers? V8's? Karts? Trucks? 1000 mph land speed record cars? Buses? Touring cars? Spec racers?
Just have a long (looooong) look at Nogrip and rFactorcentral and you'll get an idea of what's available (though not even all of it).
Personally, I think the mods of 'smaller' cars are often most fun and realistic, because they often incorporate feedback from real life drivers in those lower classes. For an F1 or CART car, input of real pilots is far less likely.
So my recommendations would be mods like the Suzuki Swift Cup, Caterhams, Formula Ford and Vintage Formula Vee (most fun you'll ever have with less than 100 BHP!), Civic 4G, Spec Miata... All great fun and relatively real-life-like.