Cars that I found underwhelming are unfortunately among the most mentioned in this thread.
Enzo - I was expecting something much more nimble than it is represented in the game. I've tried several well respected tuners' settings and still feel there's something not quite right with it. Shame there isn't a way to make it look more like the Maserati.
Veyron - Again, down to the understeering issues, although reverting the ride height and damper settings to those at stock does seem to temper it a little. Before doing this it stopped turning even the slightest bends on SSR7 beyond 220mph.
Skylines - OK, underwhelmed due to the loss of power over the numerous generations of the game. I was expecting 900+bhp fire spitting cruise missiles, instead we end up with rather mild mannered, sometimes unbalanced tourer.
I've also seen some in this thread that really I can't see why. The option Z only really suffers the lag if your gearbox is tuned slightly wrong (same with the HPA Golf), the ZZII is almost glued to the road with the right suspension and drivetrain settings and the same goes for the Nismo 400R.