I like good endings, happy or sad is irrelevant. Therefore I cannot vote.
This.
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Sad endings tend to be more powerful and memorable because of the emotional impact they have, especially when it's a longer game. And I definitely think games with sad endings need to exist, just to balance out all the countless "live happily ever after" ones where the protagonist saves the world or defeats a great evil or whatever.
I've only played one game that has a sad ending no matter what you do; even in the best ending only the player character and their chosen companion survive, literally everyone else dies.
Not going to name the game despite the spoiler warning in the thread title, but its ending is the most powerful I've ever encountered even though I did get that least tragic version of it. Writing and especially character building on the NPCs played a huge role in that, naturally.
One sad ending I will never get is the worst ending of Witcher 3. I've seen it, just never played it because I managed to dodge it, and it would just too gragic for me. Extremely powerful, but just too sad and with too high an emotional impact.
Happy endings are so common that I can't think of one to point out as especially good. Ultimately, they're basically about how the protagonist is successful in whatever their mission is. Sad endings have more variety in that regard.
Then there are endings that are "grey". Those I think are the most interesting, because different people see them in very different lights. Some may consider them happy, others sad, still others neither happy nor sad. To use another example from CDPR games, Blood & Wine has endings that divide opinions. What I consider easily the best (and happiest) ending many see as the absolute worst one.
Don't get me wrong though, I've nothing against happy endings; of course it's nice to finish a game (or any story) with a success. There just isn't that much to write about them, really.