In the movie City Slickers, one of the slickers was asked what were the best and worst days of his life. He described the best first, at some length.
Then, asked about the worst, he said "The same."
I can say that about a single day in my career.
From early in life, I'd intended to be a teacher. After college, when I was teaching at Embry-Riddle, I was what we then called a Young Turk. And my dean was definitely an Old Fogy. We did not, shall we say, get on well. Eventually he was able to make my job go away, thus ending what I'd worked for, looked forward to, and enjoyed.
That set my feet on a new path in computer software. For the next forty years, I did what I now know I was born to do. In software, I've done fifteen gigs: three as a documentor, five as a developer, and the rest in test automation. Got paid pretty well, too.
So sometimes, a defeat can be the beginning of a victory. (Always look for opportunity.)
What about your best/worst?
Note: See also the wonderful Somerset Maugham short story "The Verger."