I only use production cars; I use these 90% of the time:
Ferrari 458 Italia...
Ferrari 430 Scuderia...
Lotus Elise 111R...
Honda/Acura NSX...
Mazda RX-7...
Tommy Kaira ZZ-II...
Mazda Miata MX-5...
Nissan GTR...
Mostly light and 'fun' cars to drive with predictable handling and handle power upgrades very well. Honda S2000 is a car that I liked when it had modest power upgrades but with max upgrades, it gets more difficult to handle as it has traction/grip issues when modified too much...
Online a lot of people use the Audi R8 which I haven't played with much; very easy car to drive fast and very good for it PP level... Corvette ZR-1 is a car I'd like to use more, bit of a handful if you max out its power, so I want to take some power off of it before I use it online... Lexus LFA is another car I'd like to use that I race against a lot but haven't gotten it out of my garage more than a couple times...
I like the RUF 3400S, RUF modified Porsche Boxster but its not too competitive in its PP class so I use it offline mostly... Lotus Evora and McLaren MP4-12 (?) are cars I'd like to use more but difficult to use with sports soft tires or less, imo lack of front downforce really hurts these MR (mid engine, rear wheel drive ) cars; front end on these lose traction too easily so they can't handle corners at near as high speed as they should.