There are two different Sigma 70-200 f/2.8 lenses. This one is a macro lens and the other is an OS lens with a much faster focus motor.
What are you going to be using it for shooting?
The Canon is probably going to be the sharpest. The Tamron is the lowest on the totem pole. I have the Sigma 70-200 f/2.8 OS and the Tamron 70-200 f/2.8. I love both lenses. I shoot with a girl who has the macro capable Sigma and she absolutely loves it. Her images are tack sharp with it, but she does feel the focus in sports a bit. My Sigma is great for everything except macro. The Tamron is great for everything except sports and anything you need fast focus on.
To achieve the sharpest possible focus with the Canon you will need to shoot at approximately f/7.1. If that is a problem for you... then look at the Sigma's
My sigma is great at f/3.5 and I generally am shooting at about f/4 if I can.
The sigma macro or the tamron OR the Canon are great. If you are using flash and needing the lowest possible f/ for DOF isn't necessary I'd go with the canon. If you want to use a lot of natural light when possible and/or you need to keep that f/ down for DOF I'd read long and hard on how sharp the f/4 is or isn't wide open. If it's acceptable to you wide open? Then I'd go there. Otherwise the Sigma would then be my choice.