So I guess that this would be World War Hulk versus the Ultron from Ultron Unlimited (The one that destroyed a country.). I remember Ultron and the Hulk having an encounter during Secret Wars. I forgot exactly what happened, but I don't think that it was that much of a fight. Ultron did take on a few heroes at the same time though, and he is able to take on the entire team of the Avengers on a regular basis (And I think that he took on the Fantastic Four as well.). I think that regular Ultron can beat the regular version of the Hulk, but I don't think that he can beat World War Hulk. Then again, Ultron can't be permanently destroyed can he?
What about Magneto and Ultron? Have these two ever fought? I think that this would be a cool fight. Now we know that Mags can pull out Wolverine's adamantium, but isn't Ultron's a lot tougher? Somehow I see Ultron taking precautions. What do you guys think?
Hulk at his most powerful is world breaker level strong, in which case, if he threw a punch and hit him, he's smash Ultron to pieces along with the planet they are standing on. If he punched and missed he'd destroy the planet they were standing on and Ultron with it.
Also at that level of rage, I think the Hulk is probably just plain indestructible and could probably shrug off whatever he gets hit with. Ultron destroyed a country, Hulk destroyed a planet, by hitting somebody else...