Specific combos
AMS Taruma Copa Petrobras de Marcas (modern TC class) or Stock car
R3E Bilster Berg Volvo 240 (combi learned here)
R3E Sachsenring DTM 92
R7 Knutstorp STCC
RF2 Nurburgring AMG (Porsche Cup, as it tries to kill me)
in general
F1 - Spa, Interlagos & Monza - lately Hungaroring is growing on me
any
Mount Panorama
Watkins Glenn
Laguna Seca
Oulton Park
Zandvoort
...guilty pleasure
FM7 Mugello vintage GP cars (but cannot get new wheel to work right, so .. that ends the secret love affair).
There are sooo many to chose from but atm I'm have a blast with pcars2, @ hockenheim multi class in the atom in vr such a blast power sliding that car around the corners against the BMW's etc it just puts a smile on my face
Assetto Corsa Competizione
Spa Francorchamps
GT3's in general
Clear weather
Time multiplayer to get a day-night transition no matter how long the race is.
Hot damn, that combination is amazing.
At the moment, I love ACC, at Spa with the Porsche 911 GT3 R 2019, race at night with all the lights, beautifull !
But maybe in one month it will be AC with the Porsche 908 LH at the old 24h Le Mans circuit.
On Xbox One: Forza 7 online 'track day' private lobby with friends, loads of fun lapping mildly tuned and well set up moderate B-S class cars like the Clio V6, 02 GTR and Cayman GT4 at the Nordschleife. Being able to change cars at will and experiment with setups makes this mode (shamelessly borrowed from AC by Turn 10 of course ). This can often be as much fun as a real trackday.
On PC in VR in ACC driving all the cars at Brands Hatch Hotlap once again to see how they've changed after the latest update and which ones I get on with best, currently, the 2019 Aston is my favourite.