For me, it depends on the book.
I don't often like Happy endings, there just aren't realistic enough for me, but the times I do like them are when a book is either. A( when the main character has been through so much crap in his/her life that you just want them to catch a break. That you just want them to be happy in the end. However, in situations like these the book ends usually ends with a bittersweet ending.
Or B( when it's a light, happy, fun beach read and the couple is just to cute not to end up together.
In either of the situations I am fine and happy with a happy ending.
I don't often like reading the hard emotional endings, because while they are always the most memorable I just can't read them often. These are so emotional and draining that I just can't read them all that often. That doesn't mean I don't like them, I do. I like them, because they remind as that life doesn't always have a happy ending. And sometimes books just have to have those endings, or else they feel fake and forced. I just don't like reading them all that much.
That leaves Bittersweet. These endings are usually my favorite, because no matter how bad the bitter is, the sweet will always give you a ray of hope. It makes you think that just because it's bad now, doesn't mean it won't get better. It shows you that with every great happiness comes great sorrow and you need to live with that. And with that you need to live for the good moments and never dwell on the bad.
However, I think no matter what kind of ending it is, whether it sad, happy or in between I need closure with it. I am fine if a book couple doesn't get together (it may take time, but I'll be fine with it) as long as the author shows as that the MC will be happy. And will lead a somewhat good life. I don't like it when a book ends with a couple breaking up and the author doesn't give as reassurance that the MC will be all right. That was one of the reasons I didn't like Memoirs of a Teenage Amnesiac. Because the ending was just to open.
And it doesn't just apply with couples, whether the book is about a repel fighting for his/her cause. Or a person trying to make a name for herself. If the ending doesn't have enough closure I will hate it.
So, tell me in the comments what kind of ending do you like? Do you like a certain ending in a certain type of book? Or do not care about what kind of ending it is as long as it stays true to the book? What do you think?