it depends on what you are there for. If your ultimate goal is a post graduate degree then the answer is never. You are competing for a limited number of openings with a vast amount of applicants, you dont have the luxury of feeling at ease. There is always something more to study, another paper to write , another interview to prepare for. If you are there , however, with only the singular goal of graduating and having as much fun as possible while doing so, then likely the last semester of your fourth year .
I'm feeling my best and it's my 3rd year, but I think that's just because I'm only now living off campus, closing the book on all my weird dork trauma, and otherwise learning how to be an adult.
3rd year was when I moved out of the dorms and into an apartment off-campus with some friends. I kept getting progressively worse roommates in the dorms lol.