I'm going to be racing the crap out of the 500pp seasonal because I love that track (backwards and forwards) and I really like all the car options in the 500pp range. Pretty much any of the 500pp tunes will probably win this race if the driver is at least semi-competent (I myself fall into the semi-competent category) but here are a few I've tried and liked:
BMW M4 Coupe (500PP SH) This one is already listed but I ran it a few times and it's sort of my benchmark. I love this car and tune.
Ford Mustang SVT Cobra R '00 (500PP SH) I bought this car because it was the second car I got in GT3 and I was feeling nostalgic. Overall not too bad, about a second or two slower than the BMW and a little dicey if you go overboard on the gas in the corners.
Ford SVT F-150 Lightning '03 (500PP SH) While I was shopping at the Ford store I couldn't help myself. This is one fast truck. It's fun to play a bit of the bully on the track since you've got about 200 or 300 more kilos to toss around. Like the 'Stang, it's picky about throttle in the corners and you have to back off even more due to the weight resulting in an extra 1 or 2 seconds per lap.
Hyundai Genesis Coupe 3.8 Track '13 (500PP SH) Here is a poor man's M4. Fantastic car for the price (little more than 1/4 the price of the M4). Not that credits are ever really an issue... I started driving this car immediately after the F-150 so naturally I was over-braking and getting slow laps, but after realizing that I could push harder it I started getting some nice lap times (very close to the M4)
Mazda RX-8 Type S '07 (500pp SH) So far this is the only car I've tested with a faster lap time than the M4, but I think I need to go back and do a few more laps with the M4 since I'm now more familiar with the track. This car is also special to me because I have it in the exact same color as the Mazda3 I used to own IRL. Zoom zoom!
Next up I think I'm going to run a few muscle cars, and maybe after that start filling in my TVR collection.