I really can't say, though if forced to choose, I'd say soundtrack wins by a very slim margin. While visuals and gameplay can be perfect, absolutely stunning things... it's the music that can push you from being impressed to really feeling the location (my example of choice there would be Metroid Prime's Phendrana Drifts theme, it just sounds like ice and cold). It's the music that can make you go from seeing how sad a scene is to shedding tears (I'll actually have to try this out at some point, but I'm firmly convinced that, say, the scene when you've cured the genophage in ME3 would lose a lot of impact without 'Vigil').
That said, another big make-or-break for immersion (in my books) is voice acting, if the game has any. If the voice acting is crap, then the game is dead in the water. Writing, as others have said, is another big factor... if you have clumsy or painfully cliche dialogue, it will take me right out of the game.
So yeah, that's my take on things. It's interesting to see how many others feel the same.
That said, another big make-or-break for immersion (in my books) is voice acting, if the game has any. If the voice acting is crap, then the game is dead in the water. Writing, as others have said, is another big factor... if you have clumsy or painfully cliche dialogue, it will take me right out of the game.
So yeah, that's my take on things. It's interesting to see how many others feel the same.