Best. Track. Ever. Hands down.
The Nürburgring Nordschleife is what I spend most of my in-game time with, easily. Sure, other tracks are nice and all, but how long does it take to memorize something like Tsukuba? Or Sozuka and the like... MAstering those doesn't even compare to the 'Ring. Blind corners, elevation changes, one hell of a bumpy ride, quite narrow track width with no margin for error, high speed corners, low speed corners, long straights, banked turns... Honestly, the 'Ring has it all, except a dull design.
The Nordschleife is just special. It seem natural, as if it was built around the existing landscape, to fit in there somehow. As opposed to other tracks, which feel artifical. Driving most tracks feels like riding a horse. A fast one with a bad temper, maybe. But, compared to that, driving on the 'Ring feels like riding on a tiger. With fire set to its tail.
The 'Ring, to me, is the ultimate test track. Not only for my cars, but mostly for my own driving skills. Racing at the limits of both the car and myself for those (grenn) hellish 73 corners is what really makes me come back for more.
Keeping your driving perfectly sharp and without error while pushing it hard for about 7 minutes, now that's a challenge. That's like chaining seven perfect laps around Tsukuba
Anyways, chances are, if you can master the 'Ring, nothing else will really put your driving skills to the test anymore. You know, tracks feel a bit like a drug. And the 'Ring is the ultimate addiction. Once you've gone there, nothing else will suffice. Or be able to replace it.
As others have said, I could very well do with Gran Turismo: Nürburgring. Hold all events there, all special events, all licenses - everything. I'd probably consider it just as good a game as it is now, as crazy as it may seem.
On my 'Ring history, though; I first started driving it when GT4 came out and I hated it at first. Granted, back then I wasn't as much into cars and the automotive world in general as I am today. At first, I had no idea what all the hype about this stupidly long, back-road-ish track was all about and I couldn't stay on the track for a quarter of a full lap.
But, somehow, it always managed to make me come back for more. For more punishment of every error I made, moreso than any other track. Punishment for overconfidence in myself. I came back for more of the humiliation that no other track ever gave me, And somewhen, I finally overcame it. I learned the track. I learned to love it.
Surely, the 'Ring is a gruesome and unforgiving mistress at first, as long as you don't know how to treat her right. But, once you really get your head around it, she'll satisfy each and every of your dirtiest driving desires 👍
And, boy, am I glad that the 'Ring's back in full glory. If there was one gripe I always had with Forza 3, it was its watered down version of the Nordschleife.
Okay... This is like a testimony to my love of the Nordschleife. I'm sitting here, at work, with a tight schedule on my back and I'm still typing out such a long winded love confession to a race track

And now I can't wait to get home to get another Nordschleife fix.