It's more than likely the Nurburgring.
But that's also because it's by far the longest at over 20km.
But, the road quality on the Nurburgring is among the worst.
With that said, the other track with the best combination of technical sections and high speed sections is Suzuka by far.
It has the start/finish straight which is fairly long, after that you immediately have 2 slightly banked and technical right handers followed by a section of esses (left, right, left, right, left). After the esses you have a high speed corner which then leads to another technical part, Degner, which also has some banking on it's 2nd corner. After that, a small straight followed by a slight right hander that leads to a hairpin. After the hairpin, you have a long high speed section until you reach spoon, which is one of the hardest corners in a race track to get it right, after that, another long high speed section with the famous 130R, which for GT3 cars and the like does need to have some sort of lift or even brake but that it's flatout to things like LMPs or F1s, and after that, to end the track with a chicane.
This track is the most balanced of them all. It's a bit above average in lenght, at about 5.7km. It has 3 long high speed sections which are great for drafting and getting close to other cars or best of all, overtake them, 4 very technical sections in T1, Esses, Degner and Spoon and 2 hard braking points that are also good overtaking spots, the hairpin and the chicane.
The track also has some decent elevation changes and banking, so it's the most balanced track of them all, and the track (after Nurburgring) where you can fully test a car's performance.
Spa is too high speed focused, Mount Panorama is also very high speed focused, and also like Nurburgring, doesn't have the best pavement and it's most technical parts are way too narrow.
Le Mans and Monza are obviously way too high speed focused. Outside of the oval tracks, probably the tracks that require the least amount of aero performance.
Lago Maggiore (a GT original track) is pretty balanced as well, but it doesn't have neither the technicality of Suzuka or it's high speed sections.
Laguna Seca is a pretty short track and lacks high speed sections... In fact, I don't think we should even include tracks that are less than 4km in lenght. They are just too small to gauge a car's true performance because they are either too technical or too high speed focused. Brands Hatch is just another example...
So yeah, it has to be Suzuka.
And I would like someone to argue otherwise.