Apocalypse the concept, sounds good and all. But when it comes down to it, even as the Alpha/Omega mutant with Celestial tech and impossibly ultimate goals and intellect and crap, he ends up being nothing more than a huge letdown. For all his power and preparation and 'majesty,' he just constantly loses. He just loses all credibility. Kind of like Darkseid. Oh wait, those are just avatars that get their butts kicked.
Shiva has no personality. Doom would just immobilize one of them or redo its programming into an endless process loop so its not destroyed and therefore, won't resurrect/reprogram itself Doomsday style. Or worse, Ultron would take over Shiva and add it to himself. Ultron is a sweet addition to this fight though. If this is classic ruthless Ultron, with adamantium shell and limitless energy nuclear core, I'd say it'd come down to him and Doom. I think because Ultron has had many copies and some have been shut down rather easily, he's underrated. But classic Ultron was unstoppable. Used to fight classic Thor and Iron Man.
My money's still on Doom. Doom knows how dangerous Ultron is. He was especially wary of the Ultron in Secret Wars. He'd fight cautious and find a way.
