Every single Imagine Ubisoft game for the DS when I was younger, and definitely Tomodachi Life. I wish I could sell all of those stupid Ubisoft games, but they've gone through the washing machine, and even though they still work, I can't sell them if the pictures on the cartridge are messed up.
I'm gonna get a lot of hate for this, but I deeply regret buying The Legend Of Zelda: Ocarina Of Time. I know that's a lot of people's number 1 favorite game, but I just didn't like. I much prefer Four Swords.
Guitar Hero: World Tour for the Wii. I was young and didn't realize there were more fun and cheaper rhythm games like osu and Love Live! (etc.) out there. Also, absolutely NOBODY bought Guitar Hero for the Wii, so I could never find anyone online to play with...
Professor Layton and Scribblenauts are truly the only games I've ever really hated playing. Thankfully I got the two PL games for 5 each and I think Scribblenauts was pretty cheap too.
the last game i bought that was highly disappointing was fragile dreams, i don't really know why but i just couldn't get into it. the voices also somewhat annoyed me : _ :
Pokemon Mystery Dungeon: Gates to Infinity. It looked so awesome when I first saw it, but between the disappointing story line, the nice but confusing graphics, and the few starting Pokemon (only like 5, all the others had at least ten), I regret buying it. It's even more disappointing because the other games were so awesome.
I didn't actually pay for any of my games because I never have money for them, but I regret getting Sticker Star. The story isn't as good as the other Paper Mario games (As much of the story as I've gotten through so far, that is) and I hate having to collect stickers all the time in order to be able to attack things. Also, both my little brother and I have not bothered to even finish the story. Maybe one day I'll try to finish it, but I really don't want to.
On the other hand, there are games my brother and I never asked for but were given anyway... like M&Ms Kart racing. Stupid cheap Easter present game...
I don't regret buying it because I absolutely love the game, However I do regret buying Professor Layton when it first came out because now the price is less than half of what I paid.