Best that i can tell right now on the spur of the moment: Still Life.
I like open ended endings, especially when they're spooky.
But thinking about it a bit more got me Indiana Jones and the Fate of Atlantis and Monkey Island 1. It's an incredibly hard question, though.
Worst... well, I'd say Schizm 2. The story is horrendous all the way, but the ending beats it all; and what makes it worse it all the puzzle suffering that you have to go through to see it.
Aura's ending is so sudden, it's pretty bad, too.