Canon 70-200L f4 IS or also consider the 70-300L IS for that extra reach and if you have the extra cash.
Honestly, the sharpness doesn't compare. And if you just add a high-quality 1.4x TC to the 70-200 f/4L IS you get 98-280 f/5.6. So you get practically the same coverage without speed loss, while paying less and getting more.
Honestly, the 70-300L seems to me was a brainchild of the marketing department, not the tech department. Yes, its a large improvement over the 70-300IS, but it will attract more 1st-time L-buyers rather than people who know what they need/want... especially the 300mm end. Same as the 70-300IS before it.