What would be your apocalyptic event computing choice?

Pencil and paper and a bunch of old school reference books.

In the situation you describe, water systems in the US West will collapse, because they depend on regional electrical power for pumping. There will be a mass migration to places with food. Lots of people will become farm workers. (Not farmers - hired or even forced labor for farmers who no longer have access to large farm machinery, pesticides, etc.)

Where I live we'll still have hydro power. Most people will be working the fields growing wheat, barley, root vegetables, etc. My expertise as a software developer will be useless. I'll likely end up working the fields, or slopping pigs, with no time for programming.

The exception is that I might be able to work as a computer (in the "Hidden Figures") sort of way. Though now that I think of it, I make a lot of calculation errors.

The biggest problem is displays. Building a simple CPU is not too hard if you have the components and schematics, but how do you build a CRT or LCD without a huge industrial manufacturing backbone? We would be back to LEDs, maybe some handmade nixies, and entering code in hex or octal. Computing would just be OVER in a few years without modern supply chains and infrastructure.

Also, what CAUSES the collapse? Solar flares could wipe out a large portion of tech on earth very quickly. World nuclear war as well. Those could set us back many decades or centuries in terms of technology, and that vulnerability will only get worse the further we advance. If you're expecting such an event, a safebox full of microcontrollers and equipment with a faraday cage might be a good investment.

Assuming I have solar panels still, I'd likely do what I'm doing now: checking out BBS's, reading articles, Anki training, programming on some hobby projects.

Watching videos and listening to youtube music would likely not be possible due to lack of infrastructure, so I'd download what I can and listen sparingly.

If I'm not a farmhand and get to continue with my current job, then really nothing changes for me. I'm only limited by the sun, and the batteries in my laptop (and any spares that I charge when the sun is up)

If I am a farmhand and if I'm not exhausted after work, I might do some small web projects. It depends on how plentiful my batteries are and whether I need to keep the fridge running at night.

Maybe I'm optimistic in thinking that things would still be okay

I’d use a Lenovo t400 or t60 laptop that is fully repairable and also pretty durable. Hook that up to a ham radio data network and you’re in business.

I’d first want water filtration system. A good knife. A way to sharpen it. Untreated wood to burn. Tinned food. I’d have no use for a computer. A walkie talkie pair might be useful though. Solar or better a dynamo to charge them.

E-paper device for viewing, due to the reduced power requirements to keep text displayed.

All the hardware we have now would just get more valuable, and better cared for as it became rarer. Laptops can run for almost a working day on a charge now, so you don't need reliable power 24x7 to use them, only occasional power.

I would want some Ham radios, solar, and some spare batteries.

Most of all I would want a good library at hand. All those ebooks would be gone or be a pain to access.

Solar powered charging rig for a laptop. All data on the laptop - nothing on the cloud. Why? Because to get to data on the cloud, the far end has to be running, and so do all the routers from me to the server. You can't count on that; in fact, I would suspect it to be rare, if it ever happens at all.

Are there any computers that stand well against an EMP?

A slide rule and a paper instruction manual for it.