I have to agree with Jerry, you didn't list the Tamron SP90 Di but it's has a slight edge on sharpness/contrast. Very nice handling and the lightest of the 100mm bunch. That said the Canon 100 and Sigma 105 are great lenses indeed and still very sharp and extremely sharp once stopped to f8-11. My first choice would be the Tamron than Cannon and lastly the Sigma, but I'm certain any one of the three will perform great and keep you very happy all of them wonderful.
The longer focal length macros will give you a bit more working distance but are still much the same as the 100mm bunch optically and then theres the wonderful MP-E 65 but this lens is ONLY good for macro where as the 100mm range are a quite handy focal length for portrait and other stuff. IMO for optical quality though Tamron SP90 Di would be top of the list, but thats just my tastes. Really all them them are very good indeed, I say have a look at the weight, handling and manual focusing of each of them in a store to see what you like the feel of better for yourself.
Hard to go wrong with any of them.