I have many, the GT-R enabled me to get the Mazda Furai, which enabled me to get a Toyota 88C, which enabled me to grind non-stop.
The Veryon tuned on SS7 is unbelievable.
The X1 makes a F1 car seem like a Mazda Demio.
GT-One - with the right set-up and gearing specific to the track, it's a monster, with the wrong set-up, it's still a race leader (mistakenly left my Indy set-up in place and still won at the 'Ring). Even better than the 908 HDi, IMHO.
For working up through A-Spec: my tuned 458 was untouchable and the bone stock R8 LMS walked anything German and was more friendly than a black lab with a tennis ball. I started with a tuned R34 and gathered funds to switch to the 458.
But I'm biased. I used the GT-One to beat the previous games so tradition demanded I do that here as well.
My 458, my ACR, my R8 v10, my McLaren F1, my Jaguar XJ13. If I were to base it solely on choosing a track and just running time trials, I'd say the ACR because that thing is immense, so drivable, so fun!
I think I own every single tuned S2000 in game except for the Spoon race version, the LM version and the 2 premium cars, the AEM S2000 and the Premium Amuse GT1 Turbo. My favorite would have to be the Amuse S2000 R1
So the Ford GT with racing exhaust sounds like a drag car, it goes like stink, and it handles absolutely beautiful. Everyone, I mean everyone, should buy one as soon as they can. Just do racing exhaust and tires. Most fun to drive car I've yet found.