(in approximate order of influence)
GP2
This really made my modding and racing interest jump to a whole new level. In fact, back in the day of GP2, I was the 5th fastest driver in the LFRS league with an LFRS champion as my team-mate
GP2 was also where I invented the technique of TC-switching which eventually spread around the leagues (after much skepticism, and people saying "Don't be stupid, it doesn't work") and was declared the fastest starter in the LFRS! That and helping Juha Viitala with his carsets made it so much fun
This continued, to a lesser extent, into GP3. And my interest in the series hit a low (after owning F1GP, GP2 and GP3 previously) with GP4 when the modability was greatly reduced, everything became so much more complex to mod and - it just lacked something that GP2 had. However with GP3 I did have many fun times with people like GP4Flo, Addie Walti and many more of the great guys of the pre-GP4 GPx community.
Live for Speed
Having been around since the first release of Live for Speed, and having spent many thousands of hours racing around the various tracks in the various cars, I must say - I'm hooking. Not as hooked as I was to GP2 - though. But I'm sure that as our beloved sim develops, it will draw me in as much as GP2 ever did.
Richard Burns Rally
Having the opportunity to beta RBR I was so very excited - I'd noted all the news items, screenshots, movies, EVERYTHING, meticulously - and now it was time to play. I was so hooked, I played for days on end, barely stopping for sleep, bodily functions or anything else! Addictive, and the best physics of any sim to date or since - end of story.
GeneRally
I was so hooked on this for several years. I still would be if I had the time and the community wasn't so (largely) n00bish now. I still mod the GeneRally forums (having been part of the community since the first week of release), but have very little in-game time now.
Lock On: Modern Air Combat
Rekindled my love for flight simming - and doing it at the most detailed level. My squadron (Moose Squadron -
http://moosesquardon.mine.nu) were a very professional group of guys, we took everything professionally and seriously in the air, proper communcation protocols and flight missions. I spent many hundreds of hours on fully planned scrims and bombing raids. And many more hours trying to nurse a damaged plane, leaking fuel, back to the airbase before getting caught by incoming Russian SU-33s who had just taken off from a nearby carrier
Buggy as hell, but still the best combat flight-sim ever - possibly the best flight sim.
F1GP
My first real sim, after having played a number of regular "driving" games that were around before (you know the ones). This really sparked my interest - and I enjoyed the game so much!
Grand Prix Legends
Well, what do we say? Although I never spent as much time in GPL as I did in other sims - I've always loved it. The detail of the physics, the excitement, the skills required, it all just made for an amazing sim. Oh, and the community - *THE* community
Jedi Knight II: Jedi Outcast
I had great fun playing this with my friends over at The Jedi Academy and I've had many fun hours of sabre-locking and force battles with the great guys over there.