Depends on what kind of stuff I am shooting....
If i want BOKEH, the big 85L comes out and there is nothing else in the canon line-up that can touch it in bokeh and focal length (Anything above 200mm is way too long)..
If I am in a studio and have enough space, i will use the 70-200L 2.8 IS. The IS helps to frame the shot and since i am shooting at F8 or smaller, it does not really matter how big my max aperture is. With a zoom lens and careful position, I can rapid take 2, 3 shots of a 1/2 body, shoulder shot and head shot without moving in and out too much. Sometimes when I know what pose I want and I want to fine tune the lighting and pose, I will leave the zoom on a tripod and leave at the same place while i go and make any adjustment (change lights or touch up model's hair). The zooming feature really helps speed things up as i dont need to relocate the tripod+camera.