This definitely is a tough one. All three open up the map to a lot of possibilities.
As a contestant, I could judge based on how I would like to continue the map. If I wanted to stay with the combine theme, I could go with Reaper's map, or if I wanted more of an open-ended urban battlefield, I could go with Redwood's. If I wanted something radically different, I could go with Finger's.
Since I'm kind of at odds with myself, I'll use some kind of scoring system and decide from there.
Gameplay:
Finger. I olny died once, and that was because I got hit by a train (oops). Not to say it was too easy, I almost died a few times, and I ran for my life as many times.
Reaper. A little too hard when the combine swarm around you with shotguns, I died a lot (hard mode).
Redwood. Nothing particularly hard, zombies are pretty predictable, after all. Plus it was too short.
Originality:
Redwood. Albeit rather implausible.
Finger. Varied environments and enemies.
Reaper. More of the previous section.
NPCs:
Finger. Plenty of them, plenty hard, pretty well done. Especially those headcrabs from the canister.
Reaper. Some of the combine who ran to locations didn't respond until they got there. ai_goal_assault?
Redwood. The antlion guard didn't break the wall until I was right up against it and he smacked me. Everything else was fine, but the antlion's more crucial than Reaper's combine.
Least Errors:
Reaper. None that I could see.
Finger. A few brushes inside of other brushes
Redwood. Some AI errors (see above), and I don't think you built the cubemaps. Plus some displacements that didn't line up, and that physics props went through, as well as a white background to one of the train tunnels.
Environment:
Finger. Lots of different places, not all of them polished as much as could be, but all of them good.
Redwood. It's different, it allows more varied expansion than Reaper's, but it is implausible that there just happens to be a hidden chamber there, and that it leads where it does, how it does.
Reaper. Still a combine train station, not much new there. It's solid, there isn't a thing wrong with the environment overall, but it isn't as exciting as the other two.
There's my opinion, I won't vote until I've read some others, though.
[Edit]
A funny thing happened while I was playing one of the maps. In the beginning when the citizens line up against the wall to be shot, the one in the middle of the first group ran to a completely different place, and then back to his spot. On his way back, he got in the way of the combine shooting the first citizen.
So he became the first to be killed, good job.
Also change the texture of the dock. Docks are rarely tile. -Facepunch
As I Lay Dying