Such an impossible question! Most of the ultrawides are for crop frame cameras on a FF camera 17mm is wide, to get the same on a crop a 10mm but most of them won't work on FF, so you're limited, and frankly the ones which do aren't really up to much.
Sigmas prime wide angles are very fast, but suffer poor border performance, even on crop frames, so unless you want to play with limited DoF on a wide angle they're a bit limited.
If you want the very best performance possible buy a Nikon 14 - 24mm f/2.8 and an adaptor, it'll be manual focus which isn't too big a deal on wide angle, and it'll vastly outperform any Canon wide angle lens currently available, especially on a FF camera. Not for nothing have there been howls of anguish from Canon users fed up with the companies inability to produce a decent competitor for this lens.