L4D2 for me, it's just that little bit better, newer better weapons (autoshotty?), adrenalin, defib, more SI...
I loved L4D when it came out but the new one is just soo much more improved.
Second one for these very reasons.
Though to be fair, they feel like different parts of the same "game" to me.
Not to say that's bad, they've both been worth every penny to me.