Generally the first decision to made is budget....
Second is what are you expectations.. including what you intend to shoot
Third is to realize there is no one tool for everything.
I've seen wonderful performance out of the 28-135 IS for a whole lot cheaper than L lenses. Good bang for buck in regards to IS zooms.
I generally shoot with primes but I do carry two zooms on occasion.
24-105L and 100-400L.
Image quality wise, the 28-300mm is not the best L out there... so wedding photographers usually stay away. If you are hiking/traveling for long periods of time and packing is an issue, the 28-300mm will be one of the BEST zooms to carry because of its flexibility.
Generally, a large focal range zoom like a 20-200mm will require you to sacrifice in other places... usually in overall image quality. The question for you is what are you looking for in a zoom... lightweight, image quality, focal zoom range, packability, max aperture, etc.. sorry.. no silver bullet.