For my 5D MKii, if we are talking about lenses we own, then the Sigma 105mm F/2.8 EX DG Macro.
You can use it for landscape
You can use it for portraits
You can use it as a telephoto
You can use it in low light
You can use it for macro (duh)
You can use it for product photography
You can use it for movies Nature
I've thought about it a lot and a good 100-105mm macro lens is on of the most versatile lenses you can have in your kit even though it is only designed to do one job. The are very sharp, a great focal length, cheap, I got mine for $300, light weight, and have great focusing rings.
All of the photos above were taken with my Sigma 105mm, also here is an album of some of my favorite shots I've taken with the lens, http://www.flickr.com/photos/66627224@N02/sets/72157633893629528/