I see a lot of comments here about macro so let me add my 2 cents.
First, you have to figure out what your primary types of subjects will be. If you just want to take photos of flowers and static objects, Canon 100mm f/2.8 macro is probably the best lens for you. There is even more extreme alternative to macro of static objects for canon. The MPE 65mm that can take 5:1 subject photos, magnifying the subject by 5 times.
If you primarily want to take photos of insects, good luck taking a "portrait" photo of a butterfly at 1:1 with that 100mm macro lens. This is why you'd need 150-180mm macro..
At those focal lengths you really have 3 choices. Tamron 180mm macro, Sigma 150mm macro and Canon 180mm macro. Tamron is the sharpest out the three but is cheaply made. Canon is the best build but not the best in optical quality and handling. Sigma has optical stabilization but is only 150mm and not 180.
Most people I know use Canon's 100mm f/2.8L for macro. It makes sense because it's the "noob" macro lens. You can't go wrong with it. Others who are more in macro insect photography use Sigma's 150mm because it has better working range and still has optical stabilization and unbelievable IQ. And lastly, the super die-hard macroists usually use a combination of 65mm MPE, 100mm f/2.8L macro and 180mm Sigma or Canon.
So it's up to you to make up your mind what you are going to shoot.