Whichever I'm thinking about at the time.
But seriously, probably the things I wish I had said or done. I do regret some moments of amazing stupidity over the years, but I'm fairly sure that for most of them, I'm the only one who remembers. The things I didn't do because of fear or anxiety, on the other hand, I'll never know about.
Then again, I might regret some of them if I had done them, too.