Mossad seems like a more daring option than MI6 (though MI6 would of course be cool too) -- but Mossad seems like more of a true third option, because while probably inclined toward CIA/USA/NATO's interests, they should seemingly be motivated primarily their own interests, whereas MI6, one might think, would play pretty similarly to CIA.
Pure conjecture of course. But I'm genuinely intrigued by what a Mossad play-through could look like, trying to balance against the two 'superpower' CIA/KGB services.
I admittedly know nothing about the Chinese Intelligence services.