The 400 f/2.8 must be the king of blurred backgrounds Although I haven't had a real go with my 200 f/2 yet (hopefully that will be tomorrow)
Actually, it's the physical aperture size (as opposed to the aperture ratio) that's most directly relevant to the shallowness of the depth of field. For example, a 50mm f/1.0 has an aperture of 50 / 1 = 50mm and thus can blur a background slightly more than an 85mm f/1.8: 85 / 1.8 = 47.2. The lowly 100 f/2, though, creates just as much background blur as the mystical 50 f/1...though, granted, with only half the angular field of view and in four times as much light.
The 400 f/2.8, as awesome as it is, just barely comes in second behind the 600 f/4: 143mm aperture for the former and a 150mm aperture for the latter.