Tsukuba for me, without a doubt. (Although it is close. I generally dislike "purpose built" tracks like Fuji, Tsukuba, Indy speedway etc. and like "organic tracks" which are the way they are because existing roads and/or terrain dictate that they by that way; Nurburgring, Sarthe, Monaco, etc.)
Eh, the Nordschleife / Südschleife were purpose-built
But I get your sentiment, all the same. Personally, I have all the time in the world for all manner of circuits, except "Tilkedromes" which are too finicky and merely represent the
designer showing off.
Anyway, I also actually hate the new Fuji. I loved Fuji ("90s") from GT4 Prologue, and now I feel it has been ruined. Funny, F1 drivers feel the same, hence why Suzuka is back on the calendar instead. Incidentally, I hated Suzuka for a while, then I "got" the second half of the circuit, and all is well. I'm still not fast enough through Spoon and Degner 1, though.
And who said Monza? Yes, the first chicane is a pain; the track was originally designed with no chicanes, making the Curva Grande and the first Lesmo quite a challenge. The Lesmos are still challenging now, and just different enough that they're fun. The Ascari chicane, though, is wonderful (an example to all other chicanes!), and the Parabolica an exercise in precision. Great circuit.
Cape Ring is fine, although I hate the hairpins and that triple-apex atrocity on some variations. Periphery is great, but the jump is a tad silly - I don't mind the spiral at all (it has subtlety), except that the AI sucks at it.
SSR7 is good for what it is, not as a race "circuit", although Avusring springs to mind...
Tsukuba is technical enough that it can grate after a while, but it's not a bad circuit. The same can be said of Madrid, although in some configurations there are some truly superb corners.
Finally, the ovals. They're a separate challenge, it's about close racing (really,
really close) so it's more about learning every inch of the tarmac. The subtle differences in certain parts of the corners can be exploited by knowledgeable drivers, and it's fun finding optimal lines through the curves and often educational for car balance in general.
Some of the course creator, er, creations are abysmal, but I've made a few crackers, too
EDIT: I think the biggest problem with all of the circuits, though, is that in A-Spec, you have to battle your way through partially-witted opponents in some mad rush to make first place. Racing against equally skilled drivers can bring some circuits alive, and will show up the "test tracks" for what they are.
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