I played 1.6 for a few months but quickly moved on to Source, which I prefer. I tend to play Gungames the most as they're good for getting skilled with nearly every weapon, and they offer a fast style gameplay without it turning into a deathmatch akin to Unreal Tournament or something.

I'm yet to play CoD4 online, but I doubt it'll beat CS:S for me. I'm not quite sure what it is, but nothing seems to be as solid (for want of a better word) as CS. It just plays so well.