If you photograph sensitive macro subjects, the Sigma 180mm f/2.8 Macro lens, with its minimum focus distance of 47cm, 1:1 reproduction and optical stabilisation, could be the one for you. Read the Sigma APO Macro 180mm f/2.8 EX DG OS HSM review...
The 180mm focal length of this Sigma optic provides both the best and worst of lens qualities. Such a focal length gives a good working distance from the subject and a great narrow field of view, but it also means the lens is bulky and heavy. To an extent, this is offset by the inclusion of image stabilisation, but in practice it provides a benefit of only around 1EV when shooting macro images.
Optically, the lens is excellent, resolving a lot of detail with a high level of contrast. For those serious about their macro images, the Sigma APO Macro 180mm f/2.8 EX DG OS HSM is a great lens, especially as it has the dual purpose of being a mid-range telephoto.