What a great topic.
For me, it's an evening twilight round (walking of course), either by myself, or with a close friend I've played with for many years, on a course I'm very familiar with, rolling out on the course with no one else around.
It's those rounds where you don't really care about your score, you're not trying to impress anyone, you're not worried about pushing the group in front or holding up the group behind, or caddies, or your handicap, or catching up socially, or any of the other trappings of the weekend warrior golf grind.
It's also, for me, when I learn what makes my game tick, build confidence in my game, and try new things I wouldn't ordinarily try. Bad or potentially lost drive? Hit another. Really cool shot from a tricky area? Hit 2 or 3 to see all the different ways you can pull off the shot. Get my buddy to come over and hit a couple as well. Spending 30 minutes on the hole you like the most and 3 on the hole you don't really care for. Trying the crazy flop shot you don't try when you're concerned about your score. Playing 2 balls, one the conservative way and one the aggressive way to see who's the "smarter golfer", even though they're both you.
That's what I enjoyed the most about golf when I was 10 in the back yard, and 17 on my high school course, and that's what gets me back to my roots.