Best ending: TLJ. It tied up most of the loose ends nicely while leaving room for a sequel. Not many games can do that. The ending wasn't sudden and it was long and satisfying. Overall I think it's the best and most powerful adventure game ending.
Simon the Sorcerer 2 comes a close second. I loved the surprise ending and was completely unprepared for it. Pity the main character was so annoying though.
Worst ending: Still Life. This has to be one of the worst endings, if not the worst ending, in adventure game history. It was terrible and it left me with a bad taste in my mouth and I felt like bringing it back to the shop and asking for my money back. I know the developers ran out of money at the end but surely they could have come up with a different ending. I mean
Spoiler:Who ever heard of a murder mystery where you don't find out the murderer in the end? I thought that was the whole point of the game.
Most frustrating ending: Monkey Island 2. I am still undecided as to wether this was a bad ending or work of genius. It's all the more frustrating because we will probably never know what Ron Gilbert had planned afterwards.