I guess it depends on personal preference, but I prefer a zoom in the UWA-wide range, such as 17-40 or 16-35 - either of which is capable of professional level results. In the middle range (35 - 135mm), I think with Canon you're best off picking 2 or 3 out of the L primes (35L, 50L, 85L, 135L).
If you feel you need to go longer than that, the 70-200 2.8 IS II is the way to go. I've read that many even traded in their 135L for it, but it also depends on whether or not IS will be useful to you. With many strobes / studio lighting, I guess IS isn't that important and the primes will give you a bigger DoF control and equal or better results than any zoom option canon offers.