probably unreal tournament
UT was my first fps and I loved to keep playing it using all the different settings and maps which changed the gameplay, and if it ever got boring, i'd keep increasing the difficulty. At the time I was really into that game, using a computer when the sensitivity wasnt set to max felt like slow motion to me =D
colin mcrae rally 2.0 and driver deserve mentions, but they were mainly just for fun. I actually didnt complete the last mission in driver until many years later because I found the mission too hard. I think it was called presidents run.