Canon 70-200 2.8 IS II
I'll start by saying that I've used Canon's 70-200 f4, the Canon 70-200 IS I, Tamron 70-200 2.8 VC, and Nikon 70-200 VR II. All of them are good lenses. The canon 70-200 IS II however, is hands down the best of all 70-200's. This lens focuses incredibly fast, is incredibly prime sharp wide open at ALL focal lengths (most of the others suffer a little at 200mm), the colors are great, and the bokeh is beautifully smooth. If you can afford this lens, by all means get it and you won't regret it.
A lot of people dislike 70-200 2.8 lenses because of their weight. I see many wedding photographers only using this type of lens during ceremonies where they have to stay back and out of the way. Many people don't realize that this is SUCH a versatile portrait lens too! One thing to note is that on a full frame camera, 105mm renders things in the most true to life way. This means that when you use anything below 105mm, you are distorting your subjects (obviously the shorter the focal length, the more distortion you will notice), and anything above 105mm is also distorting your subject (flattening/widening things). While everyone looks pretty darn good around 100mm, some people actually look better with a little distortion, whether it's using shorter focal lengths, or longer focal lengths (it depends on the subject's features). Having a 70-200 lets you choose any of these options and find the perfect focal length for the person you are photographing. Having a 70-200 that's a sharp as a prime wide open, lets you not worry about losing any quality when using this versatile type of lens.