Although I like the EF-S lens I think at some point I'll be looking to improve on it with respect to both length and image quality. The EF 70-300mm L lens comes to mind. Another possibility is the 300mm f/4L prime lens and then carry the EF-S lens for filling in shorter focal lengths. This lens would also take Canon extenders.
All of the scenics were made with the 5DIII and the bird and foliage closeups with the 50D.