I think Death Toll because the boat comes in so far form the house and if your team is not ready for it, it can be quite difficult to coordinate your team to get to the boat before the horde gets there and significantly reduce your chances of making the house-boat trek.
I agree with Gate28 that the boathouse is the hardest due to the length need to travel to get there and the time it takes the boat to park. However I saw this video that had a strategy that might make it easier. Haven't tried it though so I'm not for sure.
I think the Blood Harvest and Death Toll are the easiest. Dead Air is pretty easy too. You just have to coordinate the team right. The Rooftop, on the other hand, I think is the hardest.
I have to go with the Boathouse. Not only is it the longest finale as far as the entire map goes but with the boat dock so far away and isolated, making it to the boat during the final mad rush is a pain.
I probably survive the least on No Mercy. It may just be because people have so much practice there though. I think the easiest for me as both survivor and infected is Blood Harvest. The farm is pretty win and there are plenty of trees for the tank and space for him to throw rocks when survivors run out in the open.
Comments
There is a rock and upon this rock you may build a house that shall not falter for it kills Tanks with not even a stone thrown.
That's what I was talking about earlier and I agree it makes the final much easier.
I found the boathouse easiest. Dead air also has cheap easy places to defend like the little room behind the plane.
My second is the farm house since, again, things come from everywhere.
The boat and airport are rather easy verses AI.