To (suffer and) die for (the salvation of) humanity.
Growing up as a sensitive (and religious) child and hearing about how this son of the Christian creator god suffered and died for the salvation of humanity and hearing the praise of this sacrifice, I could find little more ultimate than to follow in his example.
It is not simply a sexual fantasy, but an ultimate fantasy for me: to traverse unjustified physical suffering for a goal I deem worthy and one that saves even those who would enact cruelties against me.
It is not a fantasy I am aroused by, but one that consumes much of my imagination and my imaginary senses.
My less "ultimate" sexual fantasies still contain a good dose of genre fantasy, magic, horror, and sensual violence. They're less focused on dying and more on the verge of dying, such as a careful and intimate dance with knives, the blades piercing just enough to make me bleed.