[Sephie]

Techniquely it probably would be a very quick fight. All of the characters in the fight can die, they just cannot stay dead. between Mr Immortal and Multi-Man I would say it would be odds slightly in favor of Mr Immortal because a lot of Multi-Mans potential powers are useless or counter productive (IE being superconductive to electricity does helps him get electricuted, feeling total bliss does nothing to help at all). Ressurection Man could kill either temporarily though in the long run he is the least immortal

The actual dilema thhat you have all addressed is that it depends on what universe they are in: Marvel, DC, or another. According to the rules of the Marvel Universe Mr. Immortal will be the last living thing in the Universe and is the only true immortal in said universe. Multi-Man keeps coming back with different powers and their is a possibility that one of these powers will allow him to die permanently.