First, congrats on 3 great options!
As a city lover and Northeastern CS student that has co-op’d in LA and NYC, hopefully, I can be helpful here 
Going through some of your pros/cons:
While all of the traits are true of UCLA, I don’t think that NYU and Northeastern lack any of these. It’s also worth noting that CS opportunities between these will be quite similar and will keep basically every door open. I wouldn’t really count the rigor/prestige any different between these three.
To give you an idea of where these schools stand in the CS world, check out the rankings below that focus on research output. While very much orthogonal to the undergrad experience, it does tend to correlate to CS “prestige”:
As you can see there, all three do well. Northeastern comes in at 15, UCLA at 17, and NYU at 22 over the past 10 years in the US. Splitting hairs to say that one is better than another here for CS IMO isn’t a good place to focus. On the general rankings side, again these are all within about 25 places of each other. While it’s great to value academics, you’ve got 3 choices here on very good footing in that department.
Admit rate shouldn’t really come into play for choosing a college as its affected by way too many factors outside of academic strength. Even if it mattered, all of these have under 20%.
In terms of academic focus, Northeastern certainly is known more for its practical approach and that is very much tied to co-op. It’s absolutely an environmental difference on the whole, but there will be strong academically focused students there as well as those focused on both industry and academia.
Both NYU and Northeastern will have aspects of this for various reasons, but they also both offer incredible city resources that students use frequently, which as a city lover I have loved in both places. While it’s good to be aware of, I wouldn’t be too concerned. Being outgoing and putting yourself out there the first semester at any college will be good and give you options to pare down and develop closer relationships with people.
Some notes on the CS departments:
I picked between NYU and Northeastern myself for CS and picked Northeastern. Part of that was co-op but part of it was also the CS department itself. Northeastern strikes a balanced between theory and practical, while NYU’s department is a bit more theory and math focused. Northeastern also has an amazing teaching program that you can find described in the essay below:
http://www.ccs.neu.edu/home/matthias/Thoughts/Developing_Developers.html
As mentioned before, it’s also got plenty of research going on and is also very well represented in Silicon Valley today as well. UCLA of course is more represented due to geography, but I don’t know any hiring managers that would explicitly filter out grads from one but not the other. There are co-op’s all over the west coast.
Some notes on the cities:
In my experience, LA is just not the same type of city as SF/Berkely, Boston, and NYC. LA is so spread out and car-focused with very long transit times that it’s not the same style of city resources you may be used to. I’d recommend it for a 1 week visit but I would never live there again honestly.
Boston and NYC are both great cities but very different experiences. Boston is a smaller city but is super walkable, clean, has reliable public transit, great neighborhood feels, and a very academic vibe with over 250,000 college students. I personally would vote for it as #1 college city in the US easily.
NYC is also a great city - it’s a 24 hour city (Boston does tend to shut down around 11-1AM) and you can find absolutely everything. Downside is cleanliness and access to nature. While the parks are nice, getting off Manhattan by any means of transport takes time and effort given how sprawling the metropolis it is. That didn’t stop me from loving it on co-op though. I’m actually moving to Brooklyn post-grad in order to get a more neighborhood feel while having the access to Manhattan. NYU’s location is also in the heart of everything young in lower Manhattan.
Personally, I would eliminate UCLA because you want to get out of California, you avoid the larger class sizes, have great alternatives, and lose the less than ideal city.
Between NYU and Northeastern, obviously, I chose Northeastern myself. In your case, you also have a big price difference, but if you’re looking for a stronger academic focus there’s a world where NYU makes sense. That said, I think you can find those very focused academics at Northeastern too. You can also co-op in other cities in the US and worldwide if you like too while you’re there to get a taste of NYC, Seattle, Chicago, DC, etc. Lots of my friends used co-op’s this way
Hope that all helps! Happy to answer any specific questions or elaborate in any area!