Good evening to all,
Knowing that there's no perfect lens, I am desperate to find an ultra-wide angle or wide angle with a performance excellent or very good in the corners.
Here's what I found with the lenses I had or still have:
(had) 17-40mm L f4.0: Poor in the corners at 24mm and 20mm, but unusable for me becaus it's only f4.0.
(had) 16-35mm L f2.8 mark ii: Very poor in the corners at 24mm and 20mm, at 16mm it was terrible.
(have) 20-35mm L f2.8: Much better than all the previous ones in the corners and at any focal length, but with two probems: the minimum focusing distance is too long, only 50cm; and the lens is very prone to flare.
(had) 24-70mm f2.8: I always found this lens not very sharp at 2.8, and the performance in the corners was not better than 17-40.
Are the 20mm f2.8 or the 24mm f1.4 better? The lens I want doesn't exist? =) I will mostly use it at 24mm or 20mm.
absolutely forget the 20mm
or the 24 2.8 non-IS
You need modern primes or a superduper zoom.
Basically you tried almost everyone but the ones you need to try

.
24 1.4 II is very good at the extreme edges, extreme corners are not perfect but ok enough (not at f/1.4 though! gotta stop down some)
24 T&S II has good corners
zeiss is pretty decent
24 2.8 IS is apparently sharp there stopped down
17 T&S might not be bad
samayng 14mm is actually pretty sharp stopped down there is seems, VERY wide and good deal of distortion though
the samples from the new 24-70 II look decent, stopped down a bit, other than maybe the extreme corners (but extreme edges appear to be very sharp stopped down a bit it would seem)