Most painful: Broken tailbone in 2013. And I just bruised it a few weeks ago... ugh. (I am really not graceful!) The broken toes have hurt, too. But I've never broken anything else!
Most serious: I had my thyroid removed last March due to possible cancer. It wasn't cancerous, but it was pre-cancerous; so despite being about 10-15 pounds heavier than I was a year ago because it's taking some time to get my Synthroid dose right, I was glad I had it done when I did.
Other than that I've been pretty lucky. My only other surgery was for my wisdom teeth when I was in high school and I had never been admitted to the hospital until my surgery last year (and I was 25). I've sprained a few things (ankle, wrist, possibly my knee) but other than that, nothing.