I have a cylinder with a hole in it. The top face of it should be a flat plane but it isn't, even if there are no edges on it which could cause the unevenness.
I inserted an additional edge (the marked one) so that you could see what I mean.

Why isn't the face a flat plane and how can I make it flat? I tried to delete the top face and replace it with "fill hole" but that is not possible.
@ Mr Zak: A very good shortcut! But my tube (cylinder with whole) is concave (you can't see that on my picture). And if I apply your shortcut it isn't concave anymore.
Nevertheless thank you to both of you!
– theoretisch Dec 01 '16 at 13:23