I voted Grand Valley, because of 2 things: It's a very bearable 2.5 hours and the opposition uses road cars with Sports tyre, not Racing. This gives you endless opportunities to test all your road cars in the 250-350bhp range.
Of course, the same case can be made for Laguna Seca, which is just as much fun. Won that one with a stock Impreza just recently, gonna try with an Audi TTS next.
Nürburgring 4h would have been my choice for a track, but at 4 hours it's quite long already, and I can't seem to pick the right car for it. I'm always too fast. Maybe I just need to try it with lower and lower powered cars.
Suzuka was worst for me so far, because it rained for the first 100 laps and after hours and hours of driving around like on ice, it took about 6.5 hours to complete.