As others have mentioned, you've got a good arsenal of lenses already. So I don't think you "need" another lens. However, if you "want" another lens, specifically at the wide end, then I would offer two suggestions:
1) 24L - If you can get a good copy of this, it's a terrific wedding lens. I use it quite a bit as it's fast (f/1.4) and it's really nice for tight spaces. It also will have less barrel distortion than your 24-70 @ 24mm.
2) 16-35L - With the 5D MkII, I've found that my f/2.8 lenses are much more usable in low light because of the excellent quality at higher ISO's. This lens, although not as fast, will give you more flexibility and creativity since there's quite a difference between 16mm and 24mm. The other thing to remember with this lens is that at these focal lengths you can drop your shutter speed a bit more and get away with hand holding it. Your only concern would be motion blur.
The 16-35 is my go-to lens for dancing at a reception. I prefer the f/2.8 aperture to get a little more depth of field, and since it's typically dark, I've got off camera (and on-camera) flash going.
At any rate, both are good choices. The 35L is another great performer; however, based on your lens list, the 50 is pretty close, and the 24-70 will also cover this range.
A word of caution -- once you start using prime "L"enses, your zoom lenses might start getting lonely.