I'll forget I'm a hyper intelligent demigod for a moment and slum it with you mortals over this jovial exercise.

Mine was Diablo IV. Thought it would rekindle my memories of rotting away at Diablo for significant parts of my life, and if I didn't buy it at release I'd miss all the comradery and special events...biggest gaming purchase regret of recent memory. Of course I'm the type to not give up on something, so I blew past the playtime return window just enough to realize it's the same addiction of lacklustre gameplay bolstered by occasional dopamine hits that I have to kick every time a new Diablo game gets released.