I know the Nikon 14-24mm (which I haven't used, to be fair) is sort of the holy grail of UWAs, but the 14mm Canon L has the least distortion I've seen and apparently less than the Nikon so I'd take that for video. Although falloff and corners aren't great at 5K with it, the 11-16mm Tokina is still probably the most compelling UWA for anything but FF, and I've used it on the 5D at 16mm and even there it covers okay with really bad edges and it holds up mostly great on the Epic from what I've seen. Pretty sharp and minimal distortion for very cheap.
My current lens of choice (I shoot on a 5D) is the 17-40mm Canon L. Cheap, but not recommended. Soft corners and serious distortion. Small, weather-sealed, and nice build quality, though.