I think I'd pick the version of the Fixipelago that has access to Greater Reality, which is implied to exist in the letter to the omnibenevolence mailing list
though this presupposes that some vast number of possible worlds exist
if there is only the real world plus whatever AU I visit, I'd try to find the AU that has the least suffering and most upside potential

A) That letter is actually from way past the end of Sing by the light here at nightfall.
B) Yes, in this scenario there are a large number of existing worlds.

huh, good to know
in that case I think my choice is the same

Because I am incorrigible egotist, my main questions are A) does the AU need to be a non-trivial AU or is 'just as things were in canon' acceptable, and B) what level of obscurity for the source material of the AU is acceptable?
I ask these things because I'm wondering if any of my stories/MYCs might qualify as destinations.

it needs to be an AU, but it can be an AU of an obscure thing
the question we're trying to get at goes something like: if you were faced with a fictional world and had your choice of AUs of that fictional world to go to, what are some AUs that strike you as particularly much better than their source material?

Do "I made the worldbuilding much more functional" AUs count. Do I even want to go to [setting but with more correct worldbuilding] as a generalised operation?

I would tentatively rather live in a setting with functional worldbuilding, because then my instincts about how the world works are closer to true
But not if that came with significant downsides like "historically accurate misery", no
"Alternate Universe - Everyone Lives/Nobody Dies" is a canonical AO3 tag with 55k works; if I had to pick a tag to (accurately) apply to a world I was living in? that one doesn't capture everything I might want but I think it's a good start 

hmm, but i suspect that that's "canon characters saved from death" not "death itself dismantled" in most cases

Yes, I still think it's a good start? Assuming the canon characters are relevantly non-evil, to save the canon characters means blunting various bad effects, bad luck, depowering villains, etc
Not always but a lot of the time

Yeah very few people use that tag to mean "the protagonists are suspended in a hellish life from which there will be no escape" and I think if they did many people would be annoyed at their misleading tag selection

Does the fact that two of my settings are mutually AUs of each other make either of them count

Yes, it makes both of them count.

What settings are they, and which of the two would you rather visit?

They're the settings of my Mage-Recruit and Dating Dragons MYCs, respectively.
The difference being "what happened to the Deviser," the hypercosmic being who created the universe for reasons.
In Mage-Recruit, their argument was complete but rejected (or rather, was never acknowledged), which caused a cascading failure of its components and for the doubt/failure to resonate within all of them, interfering with future self-repair attempts. This event shaped much of the hyperterrain of Earth's local macrocosm, as well as causing Earth's universe to undergo the particular cosmological evolution that it did.

In Dating Dragons, they were interrupted before they could assemble the argument's components, and so they're scattered about the hyperterrain, but in a largely undamaged state. This also results in Earth's universe following a similar history, including an almost perfect overlap of recorded history on Earth itself.
I think I'd probably prefer to be in Dating Dragons, overall, though in both cases without any control over when in the timeline I appear there's a good chance I show up during the stone age and just have to suck it up.

to what extent does the isekaiing

- makes entirely new worlds / makes the worlds we go to more real
versus
- sends us to existing worlds / makes us less real?
i expect you might not be sure of the answer but this is a very important question nevertheless

I'm not sure what the question... is?
Does someone or something have to be getting less real in this scenario?

like for example, if we choose a golarion, does this mean trillions of people are sent to hell who wouldn't be if we chose somewhere else?
if there is even a risk of this we really really really don't want to choose a golarion. even if the au has had hell fixed i'd worry about retroactively making the people who experienced hell in the past
also if we choose an universe that has multiversal travel, do we get to go back to earth with this travel and introduce magic to it?

In this hypothetical, you're faced with a fictional world you do not choose, and you may choose between all AUs of that world that you know of, to select which of them is the timeline the target world is currently experiencing
So the question is what AUs you know of that are clear improvements on their source material
I should write that thread where Serg eats Rovagug
That's plausibly the best Golarion

oh i may have misunderstood the hypothetical. does this mean in that case, if we're faced with golarion, we should choose whichever au has had hell fixed in the earliest possible time?

If that's what you feel makes the best alternate golarion, yes

au where "asmodeus" was a trick by mephistopheles and hell was actually nice all along

But like feel free to think about something other than golarion if you can think of good improvements to other worlds

I mean it depends! Like, you could plausibly just give-up on it and see if you can find a coffeeshop AU or do something w/ no hell or generally a different approach to it, or a paritcular model of time travel and a particular set of time travelers...

ooh yeah time travel could work

paizo has canon time travel, even