My vote goes to Richard Burns Rally, and second goes to LFS.
1.) RBR is what made me want to buy a steering wheel set, and when I started playing with my logitech formula force gp, it just became SO alive, and then there was the czesh plugin for online competition, than there were new cars (really awesome ones) so I played RBR for a long time, and it needed constant training. Then there came the n4 and s1600, which brought rbr in to a new level of sim-racing, the game has survived for quite a few years now only on its fan-base, new mods , updates etc, and they are very high quality. The only drawback are the tracks :/ you just want more ! and the new tracks that are coming now ar not on par with the original ones, but RBR is the best bargain sim-racing game, I mean, you get SOOOO much for so little. RBR is a different level of sim-racing, it is fun, when it is taking seriously, and I think, that a steering wheel set is a must to enjoy this one.
2.) LFS is the common racers sim-racer, cauz it is as I see by the number of people playing, very enjoyable with keyboard/mouse, which is surprising.. LFS shines through with the multiplayer and its levels of difficulty, from an golf to a firebreathing gtr. LFS is not complete yet, it has its cons, but it is very enjoyable, and offers much fun at not very high cost or gear needed.
3.) pretty much GTR/RACE....and the others