Ok, so you slave away playing a game for countless days, destroying your social life and turning pale through a lack of sun. And then you complete it, but your sense of achievement is destroyed by the most pathetic, pitiful ending ever. Think Monkey Island 2, where you woke up and it was all a dream (sorry, that was a spoiler, but the game is ancient).
Anyway, how about a DC list of games that are worth playing for their great ending.
Your starter-for-ten: Portal (one of the funniest endings ever)