Best Game Endings:
Wing Commander III: Hearth of the Tiger (Kilrah exploded, woah!)
Wing Commander IV: The Price of Freedom (Tolwyn hanging in jail
)
Prince of Persia: The Sands of Time (it blows me off, back to the beginning, i liked it)
Portal (i give you a cake)
Day of the Tentacle
The Worst Game Endings:
Star Wars: X-Wing Alliance (the complete last act with the death star II hasn't to do anything with the main story of the game)
Command & Conquer 3 - Tiberium Wars (a cliffhanger in a C&C Game? WTF!)
Monkey Island 2 - LeChucks Revenge (i hate it! Guybrush and LeChuck as children in a Theme Park.)
StarLancer (there wasn't a real ending after all, it was a simple video debriefing, like in most fan-made FreeSpace Campaings)
FreeLancer (you saved the universe and now please leave us alone)
Just my two cents