What about the Canon EF 20-35mm f/2.8 ?
It's probably stellar optically but you're losing a fair bit on the wide end and I don't think it's USM - it is quite an aged lens - so I would imagine the AF would be slower and nosier than the more modern L series UWA lenses.
Another option is a 16-35 f/2.8 mark 1.
Whilst I won't disagree, if the focal length was sufficient I can't imagine marginal slower AF and a relatively noisey focusing motor would matter one bit when you're talking about wedding group shots.
Mid ceremony in a dimly lit quiet chapel, that's a different matter of course.