which would be a better lens

I am looking to buy a new lens. These are the 3 I'm looking at:
Sigma 18-200mm f3.5-6.3 DC OS for Canon
Sigma APO 70-200mm F2.8 EX DG MACRO HSM for Canon
Tamron AF18-270mm F/3.5-6.3 Di11 VC for Canon
I shoot portraits, weddings and landscape

Help me decide

What lenses do you already have? If you don't need a wide angle, the Sigma 70-200mm 2.8 is a no-brainer. Heads and shoulders above the other two.

I have a tamron 28-75 2.8 and a canon 18-55 3.5

then get the 70-200. the others are slow superzooms. the 70-200 is a fixed 2.8 and is a much better lens than either super zoom will ever be.

The Sigma is in a different class - an awesome lens.

Yah, the Sigma 70-200 is the top of that list, but it won't help you with landscapes. I hope you have a wide angle or something in the 28-70ish range (35mm equivalent) Superzooms are attractive, but often disappointing if you are critical about your image quality and depth of field control.