Well, considering I'm dead I don't think I'd be able to regret anything... because I'm dead... But if we're talking about just before death, I'll go with nothing, no regrets.
I don't have that many regrets in life but something I do regret a lot is not making a formal complaint against someone at uni. I made formal complaints about other people but the one person I really should have done a complaint about, I didn't coz I was too scared