Lifthrasil: I regretted buying Divinity 2, Ego Draconis very much, because of the 'screw you' ending.
I might have put this in as my most regrettable as well.
I bought it at first release, installed it and made my character, but I was more interested in overclocking than gaming at the time.
A heavy overclock on the PCIE frequency borked the operating system, requiring a reformat.
Because I never uninstalled the game and reclaimed my drm code, when I went to reinstall it, it said that code was already in use, so I couldn't play the game, I'd paid full retail price for.
It was only this year, or maybe late last year, that I tried installing it again, thinking the servers might be down or something, and it excepted the code.
Then I saw Larian had posted a patch to make the game drm free, and got that.
So I was finally able to play right through to the "screw you" ending. (Yeah, thanks for that.)