Back before I jumped ship I used a Canon 24-105L rather a lot… and it was pretty good, but then I bought a 24mm prime which was so much better.
Currently I am using a Nikon 24-70 f2.8 most of the time, and if I were going back to Canon I would probably get the equivalent. It is a big, heavy lump but optically it is superb and the extra stop of light is so handy both for low light situations and nice background blur.
That said, I think I am going to invest in a 24mm, 50mm and 85 or 105mm in the near future as that should cover most my basis – and for Ultrawide I have a 12-24 anyway… and the long end covered by a 70-200 2.8 and 300 4