The SLS is definately a great handling car, and a really quick one as well.
The 458 Italia is a good choice as well.
A real tipp is the Art Morrison Corvette. Try it in arcade mode, I was blown away by that machine.
The ZR1 Corvette is also very nice, although a bit more twitchy than the SLS or the Italia, but that is exactly what makes it fun.
Gallardo is another one. Very nice stable handling, lots of grip. But once the back starts moving you have to be awake.
NSX-R is an obvious choice, what it lacks in power, it definately makes up with handling. Plus you have the soul of Ayrton Senna driving with you.
From the NSX we obviously get to the McLaren F1, as it was kind of inspired by the NSX. I haven´t driven it yet in GT5, but this car just has to be good.
Btw, it would be awesome to have a time-limited test drive option in the dealership. Just select a car and have a 5 minute test drive. That would be a nice feature.