In the universe, what is the largest collision ( in terms of combined absolute momentum of both colliding bodies):
- Observed ?
- Generally thought possible ?
Thank you
Biggest meaning most particles involved or largest release of energy?
Define “collision”. Galaxies “collide” by having their stars gravitationally interfere with the other galaxy’s stars, and an actual collision between two bodies would be extraordinarily rare. Similarly, gas giant planets might “collide” with each other but only end up stripping off some of the atmosphere of each and having it go who knows where. The largest collision between rocky bodies we think there’s evidence for is whatever caused the formation of the Moon.
Collisions of opposing ideologies would have to be high up there in the list.
The collision of ideologies in World War II released a lot of energy and killed lots of people.
The collision of ideologies in the Cold War simmered for 50 years (or maybe 70, depending on what years you count), but somehow it managed to just burble and fume without ever going totally kablooie.
The current collision of ideologies between the so-called “West” and certain factions of Islamic societies has been getting rather, uh, energetic in recent years, and we don’t know yet where it’s going to go.
It’s a big big Universe, and we have all these energetic collisions going on right here on this volatile third rock.
Galactic black holes colliding.
From what I understand, one of the largest in terms of energy release are when a pair of neutron stars collide…supposedly some of the heaviest elements, once thought to have been formed in super nova were instead formed when these things crashed into each other.
I guess if you mean big as in size wise it would be when galaxies collide. I believe there are several that we are observing either about to collide or in the process of colliding. And, of course, our own is destined to collide a few billion years from now, which would be something to see if we were still here to observe it. 
As far as the “observed” part, I guess [comet Shoemaker-Levy 9](http:// https://en.m.wikipedia.org/wiki/Comet_Shoemaker–Levy_9) would be a big one. But is it really a collision when a gas giant planet is involved?
Collisions of two black holes are a tricky one to quantify: You can get energy releases equivalent to the total annihilation of several stars worth of mass… but we’ve never yet detected one, because almost all of that energy is in gravitational waves, which are extremely difficult to detect.
Yes, colliding galaxies are not that uncommon. But note that when they do collide, collisions between dense objects (stars and planets) are pretty much non-existent, i.e. not much more common than in noncolliding galaxies. What collides are the gas and dust clouds. When those collide, it usually initiates a collapse which means new stars form.