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If you have the budget, the Canon 8-15mm fisheye is the sharpest, most flare resistant, and most versatile lens for shooting 360 panoramas. If you don't want to spend that much, the version of the Tokina 10-17mm fisheye without the hood is also quite good on full frame.
The 8mm fisheye focal length is not optimal for shooting on full frame -- it's better for crop sensors. You'll get a lot more resolution by shooting 4 shots around at 10-12mm on full frame. Using 8mm means you have a lot of black space around the image = less image area. The typical resolution for 12mm and 4 around on the 5D2 is about 11,000x5,500.
The other option is a 15mm -- both the Canon and Sigma are excellent. This will require you to shoot 6 shots around plus one up (zenith), and at least 1 down (nadir). Typical resolution for 6+1+1 on the 5D2 is 14,500x7,250.
Many of the panoramas on my site have technical information about the camera, lens, and head combinations that were used: http://www.joereifer.com/words/category/panorama/360-panoramas/
Let me know if you have questions.