Interesting... This can operate via a process of elimination...
Upright triangle is right out - it's the easiest to climb.
Assuming climbing fast zombies, upright L offers zero advantages over a simple wall.
The inverted L and T walls are identical - no real advantage to have an overhang on the defending side like that. Both of those would do a good job preventing climbing (until a large enough group formed a pyramid) but you can't attack zombies at the base of the wall effectively. If enough get into that dead zone, tunneling under would be trivial. So they're out.
C shaped would mitigate the threat of tunneling, I suppose. This might be a good option.
I fail to see the point of the cavity for V and bowl shaped walls, other than forcing them to climb a little bit more and being more complex to build and maintain, as well as structurally more frail. They are otherwise identical to the inverted triangle. So they're out.
Diamond shapes provide a good visual angle to attach most of the approaching horde, but would be easy to climb if enough got into the blind spot.
The ellipse has a minimal blind spot, wide viewing angle, and is hard to climb. So funny enough, I'm going with the ellipse with the following caveat: That the wall be built in sections that can "rock" back and forth. Basically you can use this to "shake" climbing zombies off and crush the ones at the base into paste.
You would think that a bowl would work for that, but when you tilt the bowl, you actually make it EASIER for the zombies to climb up the rim into the inner hollow.
The ellipse would work way better and people on the top would still be able to defend when it rocks - just provide firing stations with seatbelts to keep them in place!
To clarify, there would be a backstop on the defender's side of the wall to prevent it from rocking backwards or being pushed over. A hydraulic system would push/rock the wall slightly outward from the top, crushing any buildup at the base and shaking off any zombies attempting to climb. Since we only need to rock, say, ten degrees for each section, no gaps in the perimeter will open up. you might even be able to use the zombie slurry as a bio-fuel starter to power generators for the defenders.