I would start with the ending to Super Mario Brothers 3...
"Our princess is in another castle...Just kidding! He he."
Then you can talk about P.O.W. when the word "congradulations" flashes on the screen.
Or how about TMNT "The adventures of the TMNT is concluded." Were the underwater swimming levels really worth that?
Or how about Shenmue 2? Shenmue 1 was amazing but 2 was just awful.
Halo2 ending was awful.
How about the awful ending of Diablo, stabbing yourself in the face. Then you get to become the main badguy in Diablo 2.
Now their is one other that I cannot recall that had various endings. I wanted to say it might have been True Crime Streets of LA. I thought it had one ending that was a ring of fire kind of like Cradle 2 the Grave Jet Li movie. It had another ending where you had to fight a dragon and succubus in the underworld or some crazy ass shit like that. It was like wtf? If anyone remembers that game respond and give me a heads up.