This is difficult, but my humble opinions:
Assetto Corsa: Always enjoyed the graphics and the handling challenge of the default and DLC cars. The modding ability helps too - there are some amazing cars and tracks out there to add in. AI could be better, but then, that goes for most.
GTR2: Goes without saying. Everything about this felt awesome. Challenging, graphically brilliant for its time, really well made and thought out. And so many features you want to have. Race in the rain in the dark around Spa, with a field of 60+ cars? Hell yes.
GP500: Niche, but I was a devotee, and a part of the BSB mod team as the physics builder alongside moonlighting as the commentator! Fun times for me, with some good people who I worked with. There's not been a bike racing sim as flexible or as enjoyable since, which is a shame.
TOCA Touring Cars: Come back BTCC. We only want your licence to have some fun with...
Grand Prix Legends: This is the game that got me and my Dad being competitive against one another. It's his childhood era, and my first time learning how to race cars which were a "minimum". No aero? OK. No slicks? OK. Extreme chance of hitting a tree or steep grass bank instead of run off? Erm... Teaches you not only how to drive fast but is a huge history lesson. You can't help respect the drivers who raced those in real life.
It's hard to pick just one... I'm already feeling nostalgic. Any other GP500 fans out there?