THIS is a tough question..only one car on only one track..there are so many wonderful cars, and so many wonderful courses I don't know if I could pick just one.
I'd love to take a US-spec Ariel Atom (complete with its 205-hp, 210lb-ft 2.0L supercharged Ecotec) around Sebring.
I'd also get a howl out of manhandling a Nextel Cup car around Infineon (the track formerly known as Sears Point).
One car I'd love to try driving is that Tesla roadster, an electric sportscar. I'd like to run that around Laguna Seca, but I"d probably drain the batteries inside of an hour so I'd need an Elise to keep me occupied while it's recharging.
For that matter, I think it'd be great fun to drive a Lancia STRATOS around the Nordschliefe. I know it's a completely unforgiving vehicle and it'd probably cost me my life, but at least I'd die doing what I wanted.
I'd also like to take an F1 flat-out at Daytona. (Even money which'll go higher on that trip--the speedometer or my pulse..)
And that's a very abbreviated list, mind you.