I know some people knock Ken Rockwell but I like his reviews as they are to the point and help you differentiate between lens quickly.
His recommendation would seem to be this Canon lens:
http://kenrockwell.com/canon/lenses/17-40mm.htm
The problem with a 5D being full frame is all the lense that suit it seem to be expensive. I think if you get a cheap one the camera will probably show up its flaws quite badly.
Greg.