Lancer Evolution X GSR. 4WD, pretty fast, not bad. I've been winning a lot of races with this one even though I haven't upgraded it at all.
458 Italia. Fast, probably the best S class; specially in earlier levels. Kind of hard to drive but not too bad overall. I got this one in my garage after I imported my LVL 50 FM3 profile, if you didn't get it for free then it's going to cost you an arm and leg. Has definitely helped a lot in early S class races.
I use the Skyline GT-R V-Spec but I've been struggling with this class because most of the A Class races feature mostly European super cars and my 1997 Skyline has a hard time keeping up with them. Good thing about the GT-R V-Spec is that it's cheap (~15,000cr) and just level up your Nissan affinity level until you get all parts free.
I'm avoiding buying cars even though I'm only level 20 and already have 700,000+ credits because this game gives you a ridiculous amount of cars that I'm afraid I'm going to spend 200,000+ credits or so on a car only to get it for free later in my career.