I would like to get a fisheye lens for my Nikon D40 to create 360x360 virtual tours. I currently use stitcher unlimited and the NN5.
I've been looking at a variety of fisheyes but not sure which one would work the best. Here are some of what I've been looking at...

Sigma 4.5mm
Sigma 8mm
Tokina 10-17mm
Nikon 10.5mm

Does anyone have any suggestions on a good fisheye lens?