I always say I would rather regret something I did than something I didn't do, but then I do sometimes end up regretting the things I've done! I tend to think though that things usually work out for the best, sometimes something can look like a bad move but it turns out to have repercussions later on that you value and wouldn't have happened otherwise. Like I wanted to move to Canada when I was in my early 20s, and I sort of regret not doing that, but my mum died when i was only 23 and if I'd moved away I would have had even less time to spend with her so it all worked out in the right way really.
It depends on what I did/didn't do. Like would you rather regret cheating on a test, or not cheating on a test and failing the semester, resulting in the disappointment of your loved ones? In this case I'd feel more regret in what I did do. Or not telling someone that everyone was planning on humiliating them after school or telling them and loosing the trust of my 'friends.' In that case I would feel regret for what I did not do. I would rather regret a decision that effected me negatively than a decision that effected others negatively.
That's a tough question! But really, I think whatever you've done in your past is ancient history so there's really not much point fixating on it. For stuff you never did, though, all you can do is wonder, What if?
When I graduated high school, I promised myself that I'd live my life in a way that I'd have as few regrets as possible. A dozen years later, I regard that decision as one of the best I've ever made. It forced me to get out of my comfort zone, get my hands dirty and really strive to reach my goals. My regrets are positive ones, if that makes any sense. It's much easier for me to live with the thought of, "Even though I failed, at least I did my best" verses, "I wish I had done this...".
Has it been easy? No. Has it been worth it? Undoubtedly!
Haha, I have done some very stupid things that I tend to feel worse about than the stupid things I didn't do. But, I guess the 'right' answer is to prefer for the things you regret to be things you've actually done. Stand up to that bully, take that risky job on another continent, kiss that pretty girl and the consequences be damned. 'Better to have loved and lost' etc. etc., we've heard it so many times we should know it by heart. I wish I was half that brave.