I'm gonna be devil's advocate here and not pick the obvious Dragon Trail/Maggiore/Kyoto trifecta.
My top 3 would be Tokyo South Inner (the one with docks section), Sardegna A and St Croix B.
Sardgena A despite being basically a big square, manages to make it interesting with slightly different 90 degree corners and that high speed blind chicane. The track's narrowness always make it a survival race. Plus great scenery and the only track in the game that feels like an actual road (looking at you Alsace).
St Croix is generally a disaster, but the B layout is a hidden gem. It just keeps the great flowing corners without the repeated 90 degree sections. Unfortunately, it doesn't see much use online so it's a bit unfair to pick it as a favourite.
I know most people hate Tokyo because it's grey and dull, but being a fan of Wangan it's not a fault for me. It's part of the realism. East layout is too biased for top speed cars, and Central layout is too claustrophobic. South is the most balanced but I chose Inner because the docks section is more technically challenging than the lottery chicane jump in Outer. There are a mix of high speed corners and low speed sections, elevation changes and the quirky pitlane entry. Plus it's all wall bound so no track limit shenanigans here. If you're a smart driver you will always do well here by capitalising on other's mistakes. Love it
Least favourite would be any oval or rally courses, for obvious reasons. And quick rundown on the others:
Alsace - too wide and too many similar vague apex corners that goes on forever.
BMB Infield A/B - not a fan of roval tracks in general, and A is basically a watered down Daytona Road.
Maggiore - would be up there without the first 4 corners going forward. Ruins the flow and prime track limit abuse heaven.
DTS - not a fan of CoD lottery element (if they remove the kerbs that would make it 10x better).
DTG - very close to making top 3, but the sausage kerb lottery in the first chicane ruined it.
Kyoto - very nice flow but the tall kerbs are immersion breaking and scenery is very bland (ironic because Kyoto is probably the most picturesque city in Japan).
Sardegna B/C - B has good flow in the first half but the last half (after the hairpin) just feels awkward to me for some reason. It feels better in reverse but still not quite right. Also the layout kinda looks like a Catalunya copycat. C layout is basically just a triangle so meh.