Worst experience: being laid off when the startup I worked for lost a deal. I loved that project :(

Best experience: my new company sent me to USA for a few days to meet the client. I've gained a lot more of confidence on my spoken English. I've didn't use it in years, so I was worried. Perhaps you wouldn't think it's a "dev" experience, but English is actually a required skill for a developer who has it as a second language.