I wouldn't exactly call it a "regret" but I voted Sanders in 2016 and would vote Clinton if given the chance again since I'm more ideologically aligned with her now. I still like them both though.
Even with hindsight knowing she lost?
Well I originally voted for Sanders and he lost the primary, so it's not like I can take the gamble on him anyway.
If I could magically
appoint the nominee, I'd pick him just to see if he would win, obviously. But it's not like my one vote can change anything, so might as well just vote for who I like better.