Games can have multiple protagonists. Just like they can have multiple Antagonists.

Also, both.

How can you place Booker so low after that ending ?

I vote for both.

Why do you believe this game has only one?

There is no way Columbia is that high. Once you get into Elizabeth's alternate realities, the game isn't about Columbia at all. Not a slight against it, but even by the time of Fink's levels the game is barely about Columbia.

Both, although I would say Elizabeth is more of a protagonist than Booker.

The protagonists are definitely Booker and Elizabeth. However, as much as Comstock seems to think he's the one on top of the pile, the Luteces (or Lutece? they're both technically the same person) seem to be the one(s) pulling the strings throughout the story.

Elizabeth is the reason everything is happening, but this game is about Booker.

So Booker.

Technically, the protagonist is just the person whose actions and decisions make the story move forward, so it's probably Booker considering Elizabeth would still be locked in a room without your actions as a player. The protagonist isn't always the main character in fiction though, so Elizabeth could qualify as main character.

Elizabeth is a heroine of the story, but Booker is the protagonist.

Don't read this if you haven't played the game.

Both; you can't have the story without either of them.

There really wasn't one you jump around so often everyone was good and bad at some point, you may as well add fink and Fitzroy to the list

Booker presses the action forward for the most part, so if I were to pick one it would be Booker. However, both of them are pretty much the main characters.

It's simple they obviously both are.

I consider the protagonist to be the point of view or center of events. In this case? Booker.

A literal definition of the protagonist doesn't mean the character has to be the character propelling events (in a lot of fiction that ends up being a narrator after all) but it does make sense for video games. I've argued on other forums that Zia from Bastion was the game's protagonist since she's ultimately who the entire narration is for, even if you are actually playing as the Kid.

Fair enough. I still don't consider it to be so, but you definitely widen the horizon on the definition.

Guys, don't think too hard about this. The protagonist is simply the main character in the story, the one who faces trials and obstacles that the antagonist throws at him/her. Booker, the character you play as and the one doing the obstacle facing, is the protagonist, and Comstock is the antagonist, trying to stop Booker and get in his way. I think it's safe to say that in most video games where you control one player, the player you control is considered the protagonist.

I understand, however, if then ending causes you to reinterpret everything you've ever known.

Booker, since you control him, though he is kind a tool, or the antaginist if you want to think about it that way.