Of the Canon Whites, the 300f/2.8 L II is about as good as it gets, when I recently purchased the 200-400f/4 the lens I sold was the 400f2.8 II, I kept the 200f2, 300f2.8 II & 600f/4 II, the 300 is just a perfect Lens, I use it mostly on the 1DX but also when I want to crib on weight I put it on the 5DMK III, brilliantly fast on focus lock, sharp as a razor, gorgeously crystal clear in the viewfinder on either the 1Dx or the 5DMK III, works a treat with the 1.4x extender, Ok with 2x converter, lighter by far than the Mk 1 version.
Jump on it Dylan777, I can't imagine how you will ever be disappointed, at least until the version III becomes available, which gives you about 5 + years of complete enjoyment with a brilliant piece of gear.