Easy Choice the EF 24-70 f/2.8 II, it is so versatile and sharp that I found I no longer needed my 24L and 50L.
easiest no nonsense choice..... that said, I often complicate matters.
I often get more pleasing photos with a fast 35/50. The biggest reasons I'de even shoot something else is due to the size and aperture. I really do enjoy using all the varieties, fast 35, 50, 85, and cheapo 28-75 zoom. I've owned the 24-70ii twice now, and ultimately, the size got to me. Very sharp for sure, but it's heavier than my 25/50/85 combined, and a good 40% heavier than my 28-75. Maybe one day I may go back to a state of the art modern 24-70 f2.8, but size and weight are painful. A 2/3's sharp lens, I'm happy with. When weight becomes overbearing, then I'll just go to a small prime.