I think this thread would be a good candidate for sticky-fication in this forum.
Ned Overend's book is definitely worth its weight in gold (although it sells for much cheaper than that at around $15.)
My personal top 5:
1)trackstand - a million and one uses. It'll help with riding switchbacks, technical climbs, pausing a minute to size up the terrain ahead, and pretty much any time you need slow speed balance.
2)Look ahead where you want to go and not where you don't want to go - probably the most basic and useful skill.
3)attack position - a beginner friend of mine absolutely refuses to get his lazy ass off the saddle, and he wonders why he crashes.
4)turning with your body - as in turning the bike by leaning rather than turning the handlebars. It's useful any time you have to make a fast turn. Super simple and almost everybody knows how to do it already. But it's a life-saving skill in California where the trails are full of loose rocks and gravelly during the summer.
5)proper breaking - brake for short 2-3 second durations when on relatively smooth patch when going downhill. Don't break on rough ground and don't brake on loose rocks and gravel. Again, can save you a trip to the emergency room.
Being a beginner is a not so distant memory for me. Given my own learning experience, I would say skills like wheelie hopping, doing drops, and bunny hoping are of secondary importance. Sure they are important to know, but for a beginner they are not absolutely essential. I mean, if you ride up to a downed tree branch and you can't bunny hop over it. You can just get off and walk your bike over. Nothing's gonna be hurt besides maybe your ego. However, if you ride off the trail off a cliff because you were looking at the ledge instead of the trail, then a lot's gonna get bruised besides your ego.