I'm not familiar with all of these invaders (haven't seen Man of Steel), but I'll make a shot at what I can.
The Attacks
Chicago could be worse. The Decepticons are giant autonomous robots, but they are around 8 to 10 meters high as opposed to kaiju sized. To my knowledge, they are durable, but apparently a bunch of them were destroyed via airstrike in Revenge of the Fallen. They are also reliant on energon, a fairly rare resource. So although Chicago would be trashed, I can easily imagine the Decepticons mostly being destroyed from a range before being isolated and starved.
New York would have a similar population. Most of the Chitauri were not much more dangerous than a well trained and equipped human, and even those giant flying worms would simply be easier targets for aircraft and anti-air weapons. Loki is dangerous, but can still be tossed around by explosives that fit in an arrowhead. Even the wormhole is shown to be solvable via nuke. Manhattan would be wrecked, but most of the damage could be contained there.
Magneto and his Brotherhood were in trouble even in The Last Stand. The lack of specific non-metallic weapons would be a problem, but it would largely come down to just Magneto and Phoenix. Both of whom are shown to be quite vulnerable if one can close the distance.
Phoenix is the lucky city. Voldemort and his Death Eaters were largely dangerous because of the scale of his universe. You know how many humans lived in Magical Britain? It's hinted to be somewhere between 5,000 and 10,000, based on the size of age groups shown and whatnot. There were at most 200 Death Eaters and werewolves, a handful of giants, and maybe a hundred Dementors. Phoenix has 1.5 million people, and about 30% of Arizonans own firearms. Honestly, the Phoenix City Police Department might be capable of handling this solo, simply because wizards in the Harry Potter universe are not that powerful in open combat. The dementors would be a problem, but have no loyalties to Voldemort. And even Voldemort uses a mortal shell, so even if he has horcruxes to rely on, he can be kicked a decade down the road again.
Pacific Rim covered the Tresspasser's attack well enough. In this situation, nukes might be used sooner, but whatever.
As for Zod and Ultron, I'm going to rely on plot summaries and whatnot. I suppose we could try to fight the World Engine and Zod's ship with high altitude nukes, meant to create radiation that disturbs sensitive machines rather than blow stuff up. Since the Kryptonians are not shown to be a military-oriented race, this could actually be useful against Kryptonian technology, seeing how it was not intended for military use. The Kryptonians themselves would still be a problem, but not threats to humanity.
Last, Ultron. To my knowledge, the heroes deal with the falling landmass by exploding it. We can manage. Probably with nukes.
Losses
Now for the interesting part. This action would probably mostly take place over a couple of days, and mostly in the USA. A couple of major cities would be destroyed, and a couple more heavily damaged. But most of the problems would be gone, again within the week. At which point people might start to relax.
They shouldn't.
First, note how I mentioned nukes in three of the above situations. This will have longterm effects. Ignoring the kaiju, setting off a nuke near the surface of Los Angeles is scary. Let's make it about 1.2 Megatons, the strongest nuke we have ready to fire currently. You would soon have over a million casualties. Plus, the radiation would be carried by wind currents north and east, covering Las Vegas and San Bernardo and going into the Colorado River. The same river that provides Nevada, Southern California, and Arizona with much of its drinking water. Say, isn't Phoenix in Arizona?
Now for Houston! Let's say the nuke destroys the landmass about two miles (3 kilometers) in the air. Thanks to the terrain and higher detonation, you have a million casualties from the nuke alone. And even though the Earth isn't getting destroyed, the area is still going to be blasted with smaller meteors. You know the meteor that hit Russia in 2013, with a force of 500 kilotons, aka a couple dozen Little Boy bombs? It was around 65 feet or 20 meters wide. Expect at least a few of those scattered around the already devastated area. And expect the radioactive waters to flow into the Gulf of Mexico nearby.
On top of the nukes, Chicago, San Francisco, Philadelphia, and New York are all badly damaged by these attacks. It'd be fair to say that over 5 million Americans would die in a few days.
Political and Military Aftermath
Now, the world would be paying attention to all of this. Both because the USA is highly visible and because all of these attacks happened at once across a wide area. Even if they weren't coordinated, it would suggest that this sort of thing could easily happen again. Meaning that they would have to be prepared.
A rush of military investment would begin worldwide. Countries that already have nuclear weapons would further invest in them and use them as a bargaining chip. "If you become our ally, we'll provide the only weapons capable of countering these threats." The European Union and possibly NATO would be drawn closer together, with the UK and France providing a nuclear umbrella. Germany's influence would wane unless it too caught up in weapons technology, reversing much of the German-centric focus in parts of Europe. In Eastern Europe and Central Asia, countries that would normally despise Russian hegemony might accept it in return for safety, especially if they can't rely on the EU, Pakistan, India, or China. Speaking of which, the remaining nuclear powers would gain a major bargaining chip over their neighbors. China's arsenal would draw countries in south-east Asia while Pakistan could lend its services to Central Asia and the Middle East. India would lack as wide of a natural following, but would get into a more heated arms race with Pakistan. Even if the two countries were at peace, they'd want nukes as a method of controlling the region.
Meanwhile in the Middle East, Israel finds itself in a strange situation. It's already widely suspected to have nukes, and would show them off in the aftermath. And as the only nuclear power in the region during this new age, it would be influential. But much of Israel's current position is due to American support. American annual Military Aid is $3 Billion, more than 1% of Israel's total GDP, the USA is by far their biggest trade partner, and the Israeli military uses a lot of American gear. Hence, Israel would both have their biggest bargaining chip yet and be in their most vulnerable state in decades. A war could break out involving Israel within a few years, and it would be destructive.
Meanwhile, countries capable of making nuclear programs will do so. Candidates include South Korea, Australia, and Taiwan near the West Pacific, Mexico and Canada near the USA, Argentina and Brazil further south, Germany and Sweden in Europe, and South Africa. Each of these countries would have good reason to invest in this diplomatically dangerous route, and could become regional powers and/or scenes of conflict. How would China and Japan feel about a Nuclear South Korea? Or China about a Nuclear Taiwan? How would people in the USA feel about having two nuclear neighbors? Could Argentina and Brazil form two blocs in South America? Could Germany seeking nukes destroy the EU?
Economic and Domestic Impact
With much of the USA devastated, economic activity would largely shift to other countries. London and Tokyo would compete for the #1 Economic Powerhouse with New York out of the picture, and the damage to the international market would also shift some focus to more local economic hubs. That could include Sao Paulo for South America, Toronto for North America, and Mumbai in South Asia.
Countriesand businesses with the option would switch away from trade with the United States. Mexico might align itself with Central and South America and Canada with Western Europe. Both countries would struggle though, simply because they have relied so much on trade with the USA, especially for exports. In general, the Western hemisphere's economy would decline even if other countries excelled in this new environment. China would search for new major partners, possibly in Europe or South America.
Within the United States, new cities and regions would rise to prominence. With California wrecked, there would be a major shift to the coastal cities of Washington and Oregon. Seattle would be the new hub of the Pacific coast, along with its Canadian neighbor Vancouver. The devastation of the Gulf Coast would harm not only Houston but also Louisiana and Florida. Texas's economy would shift further inland towards Austin, Dallas, and San Antonio, New Orleans would be abandoned in favor of cities up the Mississippi like Memphis, and many Floridan metropolises, including Miami and Tampa, would be disregarded in favor of Atlantic coastal cities like Jacksonville and Savannah.
In the North and Mid-West, things are not quite as bad. Though Philadelphia, New York, and Chicago were all damaged badly, the fact that nukes weren't necessary would keep these cities viable centers of population and economic activity. The eastern megalopolis could absorb much of the impact in cities like Washington, Baltimore, and Boston. In the mid-west, old infdutrial towns like Cleveland, Toledo, Detroit, Pittsburgh Milwaukee, and Green Bay could boom due to the relatively strong infrastructure in the area combined with migrating groups of people.
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