I'm not a medic, but my biggest regret is probably how much I stressed myself out applying. I thought the world would end if I didn't get into my first choice and now I'm here I realise I was worried about nothing and if anything I would have had a better time elsewhere.
Of people I know, several people I think would say their biggest regret was having a backup with entrance standards which were too high. Sadly you often can't predict how your results will turn out, even if you're a very strong candidate, and a few of my friends were caught out choosing, saying an AAB firm and an ABB insurance where the A for both needed to be in a specific subject. They got AAB overall but the B was in their best subject, so they ended up having to go through clearing which was really devastating for one of them.