When you consider distortion and other performance parameters, I would suggest benchmarking against the very best. And it can be fairly argued that the very highest performance wide lenses you can get your hands on are the Zeiss lenses. In Nikon mount these are called "ZFs"; other mounts exist for other systems. These lenses are very expensive though. So, I suggest looking at the performance of those; you can look at dpreview and photodo and read user reviews at b&h and other sites. Then you can compare those with the Nikon and Olympus counterparts.
The isn't a single "correct" answer: you have to decide what you need, how much you are able to invest, and how important the performance is.