Depends on which you want more:

70-300mm L IS USM - You want extra reach (300 vs. 200mm) and top image quality, in a compact and lightweight form. You don't need as thin of a depth of field at the long end (f/5.6 vs. f/4 or f/2.8).

70-200mm f/4L IS USM - You want top image quality in a lightweight form, and medium-thin depth of field (f/4).

70-200mm f/2.8L IS USM - You want good (not top) image quality and thin DOF at the expense of weight and a some extra price.

70-200mm f/28L IS USM II - You want top image quality and thin DOF at the expense of weight and a top dollar price tag.