Best ending ever? You wake up, and realize it was all a dream.
It's been done countless times already, and it still doesn't stop making me angry. When I take the time to go through you're story hoping to see where it goes, I do not want to see every single thing that happened in it rendered completely meaningless by the revelation that none of it actually happened within the story's setting.
I don't understand people can be so limited in their imagination that they cannot see how being an emperor could be fun.
I actually can see being an Emperor being fun, just not with the way that past Elder Scrolls games, and I'm sure Skyrim as well, are designed. The fun of being an emperor would come from actually running you're empire, which could make a whole other game by itself. If you did become the emperor in Skyrim, it would probably end up being pretty much like rising to the top of a faction in previous games, just with a higher position of authority. But the main reason I can't see the player becoming the Emperor is due to the issues of canon in future games. It's pretty hard to have the Emperor of Tamriel fade into obscurity like the heroes of past games, and even if you don't tell us who the player character was, since the Emperor would presumably have heirs, what race the player is would need to be defined so we know what race will continue the imperial bloodline.
As to how Skyrim will end, don't know, I'll know once the game is released and I've had a chance to finish the main quest. But I'm assuming it will end with an epic final boss fight (At least, it's supposed to be epic, I can't say if it will actually work out as intended.) with Alduin, which will presumably end with the player being the victor since I can't see an ending where Alduin wins allowing continued play after the main quest, and we know that's in the game. The details of the encounter, though, I can't say.