I voted 1-2 L4D 2
I think the characters in L4D2 were all around better, but I feel that some of the levels added new visuals to the zombie genre, such as the hurricane level, that seriously was not a good level to play at 12 am in a thunderstorm. I couldn't see crap and they kept coming and coming! As for the tasks, I felt L4D was the basic bait, but L4D 2 had a better Lure to the ending missions and even the mid missions.
I give L4D two thumbs up for getting me into this genre, but I give two thumbs up and two big toe ups towards L4D 2 for making an all around better game as a sequel.