1) Grand Prix Legends
Still damn good fun, but this led my life from about 2001 to 2009. The challenge of hot-lapping this notoriously difficult game sucked me in, but I stayed for the leagues.
2) GTP mod for NASCAR Racing 2003
Endurance racing as a team, including several 24 hour races, but the amount of time (and fun) spent testing before races was insane and probably the reason I've all-but stopped sim racing. Did a 10 hour race myself once just to see if I could, where one of the pre-race requirements was a brick for the brake pedal for the toilet trip during a pit-stop. I won in the class I was in and came in 4th or 5th in total. Great to do, but I'd never put myself through it again.
3) Football/Championship Manager
Spread over the years, so no single incarnation.
4) Geoff Crammond's Grand Prix 2 and Grand Prix 3 - and to a degree Grand Prix 4 before I saw the masterpiece that was Grand Prix Legends.