Formula 1 97, Automobilista and Grand Prix 4 for me - I can´t choose between them.
F1 97 was meant to be a birthday gift for my brother but instead I ended up putting thousands of hours into it and becoming hooked on anything historic F1, which for me is 1997 backwards in time to the fifties.
Automobilista is the best current sim for me by a long chalk, and with the historic mods and AI, I know it´ll be on my hard drive never to leave.
Last but not least - Grand Prix 4, when the 1997 mod came out it was a dream come true. It still has the best AI by a distance - check out this AI battle below..
Nothing gets closer to making you feel like you´re in a GP weekend, dynamic weather, pitcrews, right down to the TV cameras - which make the whole experience look like a real life broadcast - with none of those unnecessary special FX like motion blur, DOF etc that you don´t see on TV.
It has a fantastic community who have made amazing tracks and cars, the latter of which can be found in many other sims, rF2, AC etc and which don´t look out of place at all.