For me, it was definitely grad school. Part is that I was in a smaller tighter social circle — most of whom are still in my profession. I also shaped my career, which is a bigger influence on your life than how much gin you drank one spring break.
But the larger part is that I spent most of my undergrad years overcoming social awkwardness and enjoying being out of my parents’ house (i.e., wine, women and song — well, at least wine). I was much more serious about my career in grad school — I had an actual direction. Ironically, this freed up more time for having fun, reading classic novels and enjoying life. I even had a romantic life in grad school!