GT4 by far has the best 'ring IMO. Not exactly top notch physics, but its a blast to drive. IIRC it was mapped using GPS data.
The closest I've seen for PC is the "All Roads" version for rF. Its pretty good, but looks a bit wide in areas compared to onboard videos. Still a work in progress as well but its more than drivable.
Enthusia has the ring & pretty good driving physics - the best of any console game IMO, but its version of the track feels a bit flatter, and the graphics are more dull than GT4's. If you have a PS2 its only a fiver though.
Looking forward to trying out nordschleife in Forza in a couple of days, any other Xbox games have this track?