Welcome to the club!
I remember my first day with Assetto Corsa - I played for hours and hours only taking breaks to refill my drink or use the bathroom. Don't forget to grab ALL the Rouen layouts. Sadly, the author didn't combine all of the layouts into one big package like Virginia International Raceway. Rouen short, long, reverse...they're all good in my opinion. Other must-have tracks, in my opinion, are:
Lake Louise (50KM Freeroam and point-to-point races),
Bilster Berg,
Riverside,
Alto Tajo,
Aspertsham,
Algarve International Circuit (Portimao), there is a new Algarve International Circuit coming soon from Tiago Lima - I've been beta testing it for days. It's a fantastic improvement on an already awesome track. More must-haves:
Hope,
LuccaRing,
Joux Plane,
Salzburgring (from the AC Reboot Project website - ALL of those tracks are worth having),
Thomson Road Grand Prix Circuit,
Tor Poznan,
Maze Circuit,
Ebisu East &
Ebisu West (two of my all around favorites),
Transfagarasan (From Sim Traxx),
Paul Richard,
Akina 2017,
Mandagout-Pyregrosse,
Road America,
New Jersey Motorsport Park,
Ebisu North (for small or low powered cars),
Goodwood (the new one),
Glava Zete and
Blackwood. That should be enough to tide you over.
You'll discover which tracks you believe are worth it and which ones aren't. These are just opinions, of course. If you like hill climb types of "tracks" on regular car roads, not dedicated circuits, you've got to try Transfagarasan. It is beautiful. In addition to the re-worked Portiamo I mentioned above, I've also got beta versions of
Feldbergring &
Deutschland being built by
@fat-Alfie - he's one of the few top, top tier track modders. No release date on those, unfortunately. @Lilski is working on that
Watkins Glen as mentioned above. He's also a top, top tier track modder, so you know it's going to be killer. He's the creator of
Bridgehampton,
Riverside &
New Jersey Motorsport Park which are all A++ quality. He also consults on projects and often contributes anonymously to other worthwhile circuits. @Pixelchaser is working on a scratch built
Lime Rock. No ETA on that project either as some drama has popped up surrounding that project. I really feel for Pixelchaser - he's a really nice guy and I know this recent drama is really bringing his spirits down.