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?
This means that I think that for Hulk to defeat Ultron it would have been seen as a big deal. Hulk would appear as exceptionally angry and motivated. Most likely either him being solo or being seen as the final big gun for the group.
It also means that if it's simply a tuesday and that Hulk bumps up against Ultron, Ultron probably wins. In a solo setting it is to take the Hulk captive (for Hulk's inevitable come back win), or a group setting where Hulk is just one of the characters present and it is someone else's job to win.
If Ultron and Magneto were to come into conflict storytelling would demand that either Ultron has magneto defenses in place, and makes short work of Mags ... Or that Magneto comes in as an unknown (and likely a "secret weapon") and makes short work of Ultron.