No contest CS:S. If Halo came out on the PC and just the PC it would of been a big failure. The fact that it brought a genre to the console gamers, is why it's so popular.
Counter-Strike Source is superior in every way to Halo 2. Halo 2's got nothing, its outclassed on Graphics, Physics, Gameplay, Realism, you name it
CS and H2 are both great fps but for different reasons. CS sticks to the basic fps model and h2 has amazing graphics. but it really comes down to who wants what in a fps.
I voted Cs:S but only becouse i love cs 1.5 and cs 1.6 i realy dont like source to much but when i can choose betwen Halo and Cs i defineatly take Cs.