Oh I’ve learned too much useful, practical stuff to choose a favorite. I use most of what I’ve learned so far, and mostly for myself and my dog, though I’ve been able to help several friends along the way. But if I had to choose just one “really cool favorite,” it would have to be the duct tape “sutures” technique. Fortunately I’ve never needed to use it yet, but that is a super cool thing to know, especially since I camp in the wilderness for a living.