My favorite outdoor activity is backpacking through a forest in the spring, snow melting off, sun shining, with friends, breaking out of the trees and seeing magnificent view of mountains. And then cooking and talking around the evening campfire with friends.
All of my favorite trips have involved enjoying being in nature with friends, I haven't been much of a solo person.
That said, I guess I would say that my best adventure involved a giant leap in personal growth, one younger and one older, the first, climbing the Matterhorn, a very cathartic event for me, didn't feel I had to go around trying to prove myself (as much) anymore, then many years later, going on the JMT solo, many do it solo, but as I said, I don't do much solo....anyway I had a great time by myself, but a wee bit of any empty feeling, when you don't have anyone to 'high five' at those views, and celebrating the completion. Also, makes for a lonely victory celebration. But it was a big jump, in personal growth, for me.