70-200 F2.8L IS
24-105 F4L (Great focal length walk around lens)
17-55 F2.8IS (Wide angle F2.8 with IS)
Those are the 3 favorite that I own. I have to also mention the 50MM 1.8 or 1.4. This should probably be in every bag. The 1.8 is cheap, but can be a lifesaver in low light.
Adding on to that last note, the 1.4 has an 8-blade aperture instead of the 5-blade of the 1.8, a metal instead of plastic bracket, and moderately better optics.
I love all you prime shooters. My primes are growing on me, but it's hard to not have the 24-70 on my camera all the time because it's so convenient and tack sharp. But my 85L takes the most amazing pictures.
For me it would be:
Canon 300mm f2.8 IS L
Canon 70-200mm f2.8 IS L
Sigma 150mm f2.8 -- yes its a sigma lens and I stand by it - the canon 180mm costs way too much and is heavier!
But if you really want another canon only lens then add one of the 400,500,600,800mm lenses to that list