I'm mostly one for happier endings. Real life is depressing enough as it is.
Personal preferences aside, I've heard that it's better for games in general to have happy endings, just because it actually takes some effort on the player's behalf to get to the ending in the first place. Having a particularly hard game end on a downer could come off as a further slap in the face. How true is this? Well, the Soulsborne (Dark Souls + Bloodborne) games don't have the most uplifting of endings (though they are open to interpretation), and take quite a bit of effort to finish. Yet they're still well received on the whole. Take that as you will.
My own thoughts on the matter is that the ending should mesh with the game's overall tone. Having a super dark and serious game with a strong message of self-sacrifice end with laughs, sunshine, and rainbows would likely be a bit jarring, and vice-versa. Granted, downer endings don't have to be super sad all the time; a more lighthearted game could easily play it for laughs.
I also think an ending should be conclusive, regardless of the tone. Leaving an unanswered question here and there is fine and can actually do a lot for a game's reception if it gets the fan theories rolling. But ending a game on an inconclusive cliffhanger or sequel hook is not only a little irritating, it can come off as a little presumptuous because it assumes the game's going to be successful enough to warrant a sequel before it even gets out the door. If people wind up not liking the game and thus the promised sequel never gets made? The story's forever unresolved.
There was this one article I read a while back on the subject of downer endings, which argued that downer endings are better due to being more realistic, and also that a happy ending might preach a moral that the creator unintended. I disagree with that line of reasoning (for one, that seems to assume that people are going to take the happy ending literally; only a crackpot's going to look at a game where the evil overlord dies at the party's hands at the end and automatically equate that with the creator saying that all "evil" people are irredeemable and condoning violence solving everything. Metaphor is a thing that exists, you know), but it was an interesting read regardless. That article's
here, if you care to take a look.