Worst: Dreamfall and Still Life. Both for the same reason - they just... stopped. I wouldn't even call what came at the end of those two games "endings"; that's giving them too much credit. I enjoyed playing through both games and had high expectations of the endings, which made me all that more furious once I got there. If I've spent a load of money and then a whole bunch of hours on a single game I want to feel that I've put all that to good use. I want to feel that the game gives something back to me and that I've accomplished something at the end of the day. But with these two... I just felt robbed and cheated. Nothing got resolved; none of my theories about the in-game mysteries were either confirmed or proven wrong. I actually felt insulted that the developers would pull a stunt like that on the people who support them.
Best? I'm too worked up over the bad ones I can't think of any now.
