I have not played and game and I don't think I ever will, but the worst ending in video game history is Monkey Island 2: LeChuck's Revenge, which was thankfully ignored by the brilliant The Curse of Monkey Island.
Monkey Island 2? *SPOILER ALERT* It turns out you are not in fact the character you have been playing for the last two games, Guybrush Threepwood, Mighty Pirate (TM), instead you are revealed to be a child and you have been at an amusement park the whole time, while your nemesis, the Zombie Pirate LeChuck is in fact Chuckie, your "creepy" brother. In the sequels, they tried to explain it in a way that this was in fact the result of some voodoo spell and that it never really happened. So the ending that explained that all the previous stuff never happened, never happened. So despite the fact they tried to explain it away: Worst. Ending. Ever. I'd avoid the game altogether. Unlike the other Monkey Island games, most puzzles are completely nonsensical, with the Monkey Wrench being the worst offender.
Here's an idea for a better ending: After defeating the Zombie Kraken that was resurrected by the main Russian Zombie Nazi Pirate bad guy, you go to the White House where the Pigeon President gives you a biiiiig shiny medal which is in fact a giant chocolate coin, then there are fireworks everywhere and cheering then it zooms out into space where an Asteroid King Kong is standing on is making its way to Earth. Cue ending credits. Sounds better? Thought so
