Well as for who 'should' sit on the Iron Throne there can only really be one answer. Stannis Da Mannis, it is his by right.

That being said my bet is......the Iron Throne won't exist by the time the series ends, it will be destroyed one way or another. I can't see Dany ever sitting it as it would be far to cliche and predictable. (which George R.R hates)

Same with Jon Snow, a bastard boy who just happens to be a great fighter, gets given a valyrian steel sword, the coolest direwolf, (SPOILER)"is most likely a Targaryan Prince" and manages to rise his way through the ranks at such a young age. Might aswell have Elrond show up and name him king of Gondor.

I mean in my heart I'd like Jon to be king as he's one of the few characters that doesn't really want power(which usually make the best leaders), he is also basically Ned 2.0 at this point in the story