I just used a beefed GTR Black. I already had the car anyway, as well as a few upgrades. I just whacked more upgrades on it and it was pretty good to go. SS races were a laugh, but the Indy (I think it was?) race I had to do a few times. I kept making the mistake of overtaking the leader before the end of the race, at which point he'd just fly straight past me across the line. So I whacked nitro on it, 50%, waited behind him with a sizeable portion of nitro left over for the final dash. But the SS stages were a piece of cake. Forget slipstreaming, I kept my distance from those AI nutters and just flew past with what I then realised was a relatively ridiculous straight line speed.
I did try SSX with something else, some Citroen prize car or something, but the thing was wildly unstable at high speed and I couldn't be bothered trying to do anything about it. The GTR on the other hand was a piece of cake to drive.