Most would recommend start with the Canon 60mm, which is what I did. It is more versatile as you can back off and do fish portraits. The 60 mm also works better in low viz situations as already mentioned. It is also generally easier to use.
I recently also got the Canon 100mm (the newer more expensive one with IS). It is a very nice lens (pricey of course). The longer reach works well for skittish subjects, but of course that works only in good viz. You can also get more magnification with it with a wet diopter (because the longer working distance is cut down). Keep in mind that without a diopter, they both do the same 1:1 magnification. It is just with the 100mm, you will be further from your subject. I don't know much about the Sigma 105 mm. Some people like the Tamron 60 because it can take a teleconverter but I don't have direct experience with that.
Bottom line is I like both the 60mm and the 100mm. The 60mm because more versatile, and the 100mm for skittish subjects and ability to do super-macro with a wet diopter. Start with the 60mm, but eventually you will want to get the 100mm also! :-)