dabac

Thing is, some of those requirements are likely to be mutually exclusive.
"Toughest" would probably be some adaptation of DH stuff, with a 142x12 axle.
There'll be a limited number of shops that have those parts sitting around.


"Simplest" is probably something along the department store variety. Solid axle, thread-on freewheel.
"Most repairable" is tricky and require clarification.
There's a difference between what can be done SOMEWHERE, and what can USUALLY be done.
Take your pick between "most replaceable parts" and "easiest accessible parts/tools/skills".


There's a HUGE number of shops that'll know how to deal with cup & cone hubs.
And unless you shatter a cup, pretty much any shop with a decent parts bin would be able to piece together a rideable wheel.


A hub with cartridge bearings is easier to repair than a hub with a busted cup, but not all shops are set up to deal with cartridge bearings.



Can't help you there. It's YOUR imagination.
Me, I'd either go for a 6-speed FW, cartridge bearing hub with a high-quality axle or one of Shimano's best sellers. In slightly different ways, they'd both qualify for "easily repairable".



A disc brake hub doesn't care what rim it's laced to as long as you get the spokes right.


Can't help you there.



Bicycles generally have the choice between cup & cone AKA loose ball bearings and cartridge bearings.
Both are types of ball bearings.


Cassette wrt bicycle wheels is something else entirely.


So I assume you wan't a cup & cone hub as opposed to a cartridge bearing hub.


I think your assumption is wrong here.
Loose balls are easy to come by, it's often possible to piece together a working combination of cones and axles as long as you accept seals not lining up quite as they should.
But cracked or pitted cups often means the hub is a write-off.
While often technically replaceable, I don't know of any shops that do it.
Outside the bicycle realm, cartridge bearings OTOH are well standardized and usually easy to come by from any industrial supply store.



You need to differentiate material strength from component strength.
Yes, stainless as such is stronger than aluminium.
But all stainless rims I've seen have been single-wall.
The strength gained from the box section of a double-wall aluminium rim is more than the strength lost from the weaker material.



What kind of damage do you expect to need to repair, cracking around the spoke nipples?
I'd think a a good, double-eyeletted aluminum rim would do better WRT that than a single-wall SS rim.
Who'd do the welding?
How?
If there are fatigue cracks in one spot, odds are the other nipples aren't far behind. Reworking the whole rim would mean an unlace. The welder would probably want to be payed for his effort.
At that point, the benefit of being able to repair as opposed to replace begins to look quite thin.


Nah, weight is only ONE consideration. There are others.
I've been kinda tempted to dig up some old 700C SS rims to build a hub-brake winter commuter.
Roads over here get salted.
But corrosion doesn't kill aluminium rims fast enough for there to be enough benefit from such an effort.



While it's certainly nice to have stuff that lasts a long, long time, sometimes one really have to look at the bigger picture.
If the effort needed to avoid a future effort is bigger than the effort you're tryding to dodge, then you're better off simply dealing with the unavoidable.
When I started full-time bike commuting I used ceramic coated rims and rim brakes.
They lasted years and let me feel a bit smug about the nice bits I used.
But I worried about certain bike racks and potholes that could flake the ceramig off and cause grabby brakes.
When they finally wore out I started buying used generic wheels instead.
I lost a bit wet braking performance, a big chunk of worry, and it goes out about even in cost.
And since this approach generally meant I had another wheelset held as spares, I could simply swap it in and keep riding and do repairs later when something else happened.