28-70 F2 definitely, when Canon will come up with a serious R camera (with joystick again, please, not this annoying slide bar).
For birding, I am more and more intrigued by every new generation of Canon's 600m F/4 lenses. They are so light now that I seriously consider an upgrade of my 500 mm lens. The first EF generation of this lens was by far too heavy to transport it far in rugged terrain, but the newer generations are really close to perfect. I still hesitate because I still hope an even more compact and lighter DO version to hit the market. I am pretty sure that Canon's technology, including the required manufacturing precision, is mature enough for bringing out such a great lens.