I'd hesitate before splashing out on a fisheye, as it's a novelty lens. Sure, if used well it can create some great photos, however you're unlikely to want a whole collection of them - a conventional wide angle zoom is likely to be much more useful.
I don't have any experience with the Nikon and Canon lenses as I shoot Sony, but the Tokina 11-16 is generally regarded as a top class lens. Sigma make a few, the 8-16 (although I'm not sure whether this can take filters), two versions of a 10-20 (I have the f3.5 version, it's great but as others have mentioned the 82mm filters are expensive on this version), and Tamron make an 11-18 and 10-24. There's probably not a huge difference between any of the above, but consider if you want to shoot interiors using the lens (wide apertures are good there), or whether you want a good zoom range on the lens.