I guess it also is important to ask what type of riding do you want to watch? Freestyle? MX? Offroad? A mix of everything?
For off road you can't beat "Time to Ride". Hawkins, Abbott, Watts, Davis and a bunch of other off roaders dicing it up. I also really liked the 2000 Enduro Series Highlights from Digital Video Factory.
I picked up "Project MX - Terra Firma 7" and while it is really a high quality vid, it is too short. Not as much fun to watch as some of the others. Some of the special features were good though.