Before making a buying decision, you need to work out if a macro lens is for you. Clearly if you want to shoot macro images, the answer is yes, but how about if macro is not at the top of your list? Well, macro lenses are actually some of the most versatile around, especially when they fall around the 90mm focal length, because they make very good portrait lenses that are sharp and offer a fast maximum aperture to help achieve a shallow depth of field. Many models now also include some form of image stabilization as well making them great in hand-held low light shooting.
If you are looking for a macro lens and price is not a major concern, then the Canon lens is clearly the best of these three options. However, it is also the most expensive. The Sigma falls in the middle in terms of performance and the Tamron is a little behind that. However, while the test figures may suggest the differences are large, in the real world they are quite minimal and the differences in optical performance are unlikely to highly obvious in your final images. Given the $250USD saving of the Tamron over the Sigma lens, this addition to the Tamron range does still make a very good choice and is more than capable of performing alongside other lenses in this category.