If any people were going to fight for my enjoyment, I would pick the most outrageously over-powered people.
I would have Sephiroth (the whole teleporting stuff from KH2, and all the energy beams of death flying off of a seven foot sword from Crisis Core's best cutscene)
And I would make him fight Alex Mercer, mainly cause Alex's biggest thing's that would make Sephiroth lose is his stealthy disguises and his consume thingy, and Sephiroth is a dick (in a totally awesome badass way) and would kill anyone who wasn't Alex/Alex in disguise, and secondly, he is too fast and teleporty and awesome to be pinned and consumed by Alex.
I think the fight would be AWESOME!!! It is like, the constantly healing awesome ninja of death vs the awesome, teleporting, sword wielding, ninja of spell-casting (materia), energy-beam flinging and sword-swiping.