The older I get, the more I like happy endings, but really, anything that fits the series is good.
I definitely have one that I hate though: The
supposedly happy ending where only main characters are happy. Happens all the time in harem series(but it's not limited to them), and pisses me off. Those are not intentionally bittersweet endings - they are supposed to be run of the mill happy endings, but they're only happy by conveniently ignoring the characters who are hurt in the chosen outcome. The endings that completely defeat the purpose or spirit of the series are pretty awful too - TTGL, Abenobashi Mahō Shōtengai, or Canaan come to mind.
Oh, now that I think about it, there's one kind of ending I like - where everything goes to hell, but the main characters are together, and there's a chance that they, in some twisted and broken manner, will achieve some form of happiness. Other then in the endings I dislike, this is done intentionally. NGE(End of Evangelion rather), SaiKano or Anne Freaks would be examples.
I would be dead ashamed of myself if anyone saw me on the last episode of AIR
AIR was an example of an ending that tried too hard to be really sad, but in the end just kind of come out pointless and failed to really affect me. I'm aware I'm alone in thinking this, though.