There's the f/1.2L 50mm which is better quality than the f/1.8 version, and 2 different 85mm lenses, and the 24-105 f/4L IS, the 24-70 f/2.8L, the 70-200 IS in f/4 and f/2.8 versions...no end of high-quality options.
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These are ALL fine recommendations and each is quality glass.
Although I am partial to the 24-70 f2.8L which is my "daily driver", supplamented with a 70-200 f4L
you may find that for weddings, the broader coverage of the 24-105 f4L IS
is more suited for your use...especially is you are looking for a one lens workhorse for weddings.
With the XTI's crop sensor...I wouldn't be worried about getting into the wider lenses...like the 24-your choice zoom...
Many consider it a small issue giving up a couple f-stops for the broader coverage of a 24-105 f4L IS over a 2.8 or 1.8 lens...
And while I see their point I don't tend to agree unless one is going to also add 2 wide open Primes to their collection...like the 50 f1.2L and/or the 85 f1.2L
Part of the answer lays in the question posed by mahleu...do you shoot mostly with natural light or flash?