So many, here are ones that hit me right in the FEELS:
Tracks:
-Redbull Ring: stunning recreation, easily one of the must have tracks for AC
-Road America: not top quality yet, but the track racing surface is great, needs work at the pits and surrounding environment to really let it shine in the AC world.
-Paul Ricard HTTT: expert level mod
-Lake Louise: made for road cars, try some beast on there it's just a joy to drive.
-Lucca Ring: work of art.
etc.
Cars:
-787b
-Ford Cosworth RS500: crazy good
-Cobra SC: stunning to race on Pikes Peak leaving 200 foot skid marks in first gear
-Formula Renault 3.5: a formula racer that seems driveable, stunning recreation.
-IFR TUDOR mod: coming along very nicely.
-EGT/PX pay mods: only car pay mod I have and well worth it, maybe the EGTs are a bit easy to drive.
-Mazda Miata: best sounding car in the game. Proper use of Fmod, whoever did the sounds needs to be poked by Kunos asap.
The mod community in this game is just really talented and more stuff is in the pipeline which is really exciting.