My favs are the clips I've made myself of my rides...
I like watching my videos too, but I also enjoy videos from other riders. Especially steep sufferfest climbs with original audio, which is rare in the mtb world from what I've seen so far. Road cyclists seem to post more of those, and even then they're not common.
The reason I like videos like that is because they are inspiring. When I watch another rider's video of a tough climb with natural sound, it motivates me to ride harder on my next sufferfest. I love climbing, so that may have something to do with my preferences in videos.
After last weekend's crash on the "super gnarly" section of trail near the end of the ride, I searched YouTube for other people's videos of the same section so I could study their technique and hopefully ride that section better next time, without crashing of course.
The video in the thread I linked to above has a rider cleaning that section near the end. I also studied his technique.