I have the 100-400L, the 70-300L and a 28-300L. The 28-300L rarely comes off my 1D X! However, I am ready to purchase a true "Great White" (I know, they are actually gray). It will probably be the 300 2.8 II. Although, I am tempted to go with a 400 2.8 II, I think the 300 will best serve my needs (BIF, wildlife & motorsports).
Of all of the lenses that I have read reviews on, just about everyone rates the 300 2.8 II as probably the sharpest telephoto lens that Canon makes. It also seems that it suffers the least when teamed-up with the 1.4X III and 2.0X III TCs. That tells me a lot, as does having seen many fine images made with that lens on this site and in numerous publications.
I would also like to see an updated/upgraded 100-400L, the flexibility of that zoom range makes it a very handy lens indeed.