Can't see how people like the Fallout 3 ending, it's not that great. The "different" endings are just 4 different narrations, would've been better with no narration at all, just a zoom in on a picture with Liam Neeson and yourself.
For my favorite ending, I will have to go with... Batman: Arkham Asylum. Yeah the final boss was lame, but the big-bang punch you use to finish the boss off is awesome on so many levels that it's better than God.