Ok well this one is easy for me, and it has nothing to do with the game itself but more to do with where I was in life at the time.
All time number one: Gran Turismo 2
Reason: Had many, many, many misspend hours of my youth playing this with mates, all crammed around a small 24" TV doing time trials at the various circuits and trying to beat each others best ghost times. Countless laps at Laguna Seca have a special place in my mind even to this day. Ok it was back in the day before the likes of us had internet access, so I've no idea where we would face on a world leader board, but the way we pushed and pushed those cars I'm seriously sure we would feature in the top four or five places with ease. Properly good times back then, mixed in with lots of......... intoxicating stimulants
and a good time was had by all.
Truly some of my best moments with a virtual race car, despite the obvious limitations of the software and hardware (we didn't even have a wheel and pedals!). Epic.
Coming in hot on GT's heals in second place would be Forza, and basically for the same reasons as above. One of us picked up a new Xbox and we tried it out and fell in love, quickly rejecting good old GT and migrating lock stock over to the Forza branch of racing games. It's like back in the 60's when you were either The Beatles or Rolling Stones fans I suppose, you just had to take sides with one or the other.
Years later when I got married and bought my own house, we put internet in and I discovered racing online with Forza (forget which one, probably number 2?), joined a small clan of sorts called Team Mature Gaming and had myself some pretty epic races over a short period. All got spoilt one day when I discovered rFactor, bought a PC and haven't touched the xbox since!
Rounding out the podium in P3 would have to be Race 07 for me. Although it wasn't really that good compared to it's PC cousin, it in my opinion marked the first step into realistic simulation of a racing series for consoles and for that reason only it deserves big credit. Shame it never took off, as the PC game is mind blowing how good it is.
4th comes WRC 1, because at the time on PS2 it felt superb and was the best rally experience I've had (at the time!) and provided hours of fun with some mates, plus the graphics looked really awesome for a console and it really did do a good job of making you feel part of a real rally.
In 5th would have to be F1 97 for the original PlayStation. That was the first F1 game I owned that felt realistic (I had the old 80's ones before and some of the PC ones too), but F1 97 felt like the pinnacle of racing simulation at the time - it even had the proper timing sponsors and as real commentary!
Great times all of those games represented, and to be frank I often think about what we have now, both in terms of software and hardware, and just feel so lucky we live in a world where this is all possible. I've said it before and I'll say it again.. sim racing is awesome people!