Heard Canon has a new "W" series of lenses designed specifically to shoot weddings.
They do everything, including :
Reliable metering even when too many white(dresses) or black(jackets) subjects are present
Super-fast USM for tracking of couple's movement
Perfect WB without color cast even when shooting in funny tungsten lighting
Maybe you'd like to check it out at Canon showroom
It depends on your shooting style. I will get a fast prime and a f2.8 zoom. Sometimes 85mm f1.2 and 16-35 f2.8. Other times 24mm f1.4 and 24-70 f2.8. 70-200mm f2.8 will be in my bag depending on the location i'm shooting in.
Any lens is suitable - but unless we know more about the shooting location, the conditions there or your personal preferences in style and outcome we can only guess. It's the same as asking "Which car is the best to bring the couple to ROM?" Some people use 10-22, other 17-50, 24-70, 50, 85, 70-200 ... take what you need for the situation. Also, your camera (crop body / FF) will also affect the lens selection.
any lens has its place. even a fisheye or a tiltshift.
the problem is, you will need to compromise between bringing all the lenses or using all the lenses and consider the fact that you will actually have problems switching constantly to get shots you want.
that said, doesn't mean you go and start buying 18-200 lens with teleconverter + wide angle adaptor + fisheye adaptor.
Most experienced wedding photographers will use 2 cameras, one with a wide angle/standard zoom such as a Canon 17-55 f/2.8 or a 17-40 f/4 or a prime such as a Canon 35 f/2 or similar, and another camera with a zoom such as a 70-200 or similar.
If you are supposed to be the main photographer... I suggest you'd better tell the couple to hire a real professional to shoot the AD instead of relying on you. You really don't want to take the responsibility of screwing up the couple's one-in-a-lifetime wedding photos until you have some experience as a backup photographer understudying the professionals for some time.
If you are going as a backup photographer, it doesn't really matter so much what lens... A fast 50mm or wider or a f/2.8 standard zoom are good choices if you have one of them.
Most professionals I see carry two bodies, one with a wide angle zoom lens, and the second one a fast normal or short telephoto lens.