Hard to pick just one.
I guess for me, I spent a huge amount of time on Revs on the BBC computer at school, before finally getting on my C64.
GP1 through to GP4 I also spent a lot of time with. Then came Grand Prix legends, a game that felt so rewarding to get right, I don't think I've ever felt so driven to get good at a sim since then, largely due to GPL rank.
Indy 500, Indycar Racing 1 & 2 and the Nascar series. Had them all, played them all to death.
GTR2 was good, excellent in fact, but I don't get the love for the AI. They were too easy to beat in long races, because even if they had a 30 second lead you totally could destroy them in traffic.
AMS1. Probably the best ISI based game for me. Looking forward to see what AMS2 brings.
AC and ACC are probably my current favourites, I think ACC is going to be regarded as a classic that we'll be talking about for a long time. I'm hoping that AMS2 gives it a run for it's money though, just because of the varied car roster.
If I had to pick just one of those, it would probably be GP4, with GPL a very very close second, but I'd be keeping an eye on ACC and AMS2.