Well which lenses do you have? Or are you looking to buy one? Have you considered renting one?
Depends on budget? EF-S 17-55 f/2.8 ?
+1 this is a great general purpose lens and a huge step up in image quality from the kit lens.
Plus it's got a pretty wide aperture meaning it will be fast if the wedding is indoors and you aren't allowed to use flash.