So, I mentioned this idea a few days ago and I'm finally getting around to it.
The idea behind all this is a sort of card matching experiment. Specifically, what our characters go to when things have just gone fubar and desperation calls for the hail mary pass. When the chips are down and the dice have already been tossed. That moment when the gloves are off and.... well you get the idea.

This will be an exploration, not only of the cards our characters uses, but what they use in their darkest hour.
It's a sort of interesting thing, I think, because at it's heart, it isn't only about what the characters' most potent spell is, but also, rather, what they might be remiss to use unless they have no choice. You can tell a lot about a person by what they hold back.

If you don't mind, I'll get this started with a little cross selection of some of my characters:

Cara: Eternal Dragon
Because as she likes to say "Humans aren't the only things that leave ghosts."
Alessa: Depending on the circumstances either Warp World or Beacon of Tomorrow
Sometimes you need a little more time, sometimes you just need the problem to be something else. For the sake of full disclosure, Warp World would have a terrible effect on Alessa. Chaos Warp is usually enough to knock her on her ass, Warp World might put her in a coma.
Raef: Runed Halo
We've actually seen him pull this one out of the bag before. I really like how this kind of underlines that he isn't capable of very expensive magic, but can manage a few fairly potent spells all the same.
The Shifter: Treacherous Urge
There are times when one well placed betrayal is sometimes worth all the Insurrection in the world. And after all, there's such a delightful symmetry in having a spell that cleans up after itself. (Plus I just freaking LOVE that art.)
The Duchess: Worldpurge
If one is losing the game, it doesn't mean it is time to quit yet.

I'll probably sweep back in and do a little more this and that as we all get to see some of the other character's pocket aces.

Lumina: Terminus. Her ultimate power is the ultimate truth - at the cost of destroying everything she ever cared about.

Auric: Immortal Coil. Auric still remembers the man he once was. To lose his past is to die forever, subsumed by the gold that claimed his heart.

Illarion Vale: Illusionist's Gambit
Illarion honestly doesn't have the most going for him -- he's not a hugely powerful mage, and in a pinch his little drops of pyromancy won't avail him as much as playing to his creativity to divert the enemy and get out of there.

Amah al-Hazid: Juzam Djinn
When things have gone totally pear-shaped, conjuring a very surly being of immense power can't make your situation THAT much worse.

Dantalion: Postmortem Lunge
Dantalion is somewhat interesting in that he's a schemer and manipulator at heart, and while still magically potent is quite weak if forced to abandon his ready hosts. Of course, he can infest corpses. That has a lot of disadvantages, but if it's available, the biggest reasonably intact dead thing can serve as a last-ditch vessel.

Ilyria: Retribution of the Meek
Not all of Ilyria's overthrowing of tyrants was by pure force of Banners Raised and True Conviction. She was (and might still be) a powerhouse in her own right.

Kala: Platinum Angel
There's not a great card for Kala. Boros Charm, Mode 2? Indomitable Archangel? Neither are really in her colors, though she does have 5c mana access, she REALLY prefers blue and red. When Kala's back is up against the wall, her first priority is defense, getting even a temporary shelter for her (and any friends) against her foes. Angel's Grace, again, troublingly white. White's just so good at saving your rear when crud hits the fan. In the end, I went with Platinum Angel because I think it gets the idea across, though I'm not clear on how an angel construct works to protect you from all forms of defeat and demise.

Lia Xin: A Gun
We've seen it in action! If she really needs to save her own rear, Curtain of Light is probably her best.

Lourima Viiran: Death Cloud
Lourima is all about a massive outpouring of black magic. Drain Life/Exsanginate, that sort of stuff. But if she needs to be 100% pure destructive, an outpouring of raw kill seems very fitting.

Mari Gwynn: Holy Day
Mari has more magical potency than even she realizes, but most of what she uses is bent strongly to healing. So, naturally, her panic button is in the damage prevention sphere.

Mattias the Tinkerer: Tinker
Mattias is first and foremost an artificer. The magic he's learned from Kala mostly gets things done faster and more efficiently. So if he's in a jam, he wants to grab the artifact that gets him out of it, not use a spell to get out directly.

Narissa Enshar: Drain Life
Her best move is to empower and heal herself by killing off something else. Momentous Fall, Weed Strangle, and Exsanguinate are also good touchstones and good panic buttons.

Sorinne Datharius: Wild Guess
Sorinne's a decently spontaneous person. If she's got nothing, she might as well pitch it for the chance at something.

Ellia the Endbringer: All Is Dust
Ellia is, somewhat like Dantalion, a manipulator, but she's packing an order of magnitude more sorcerous power and stockpiled goodies than Dantalion could dream of. If she has to enact brute-force ruination on a plane, she can do it. Apocalypse was a serious contender, despite its redness -- I especially like that the "discard your hand" term could be code for how much research Ellia stands to lose if she just flattens a world for one reason or another rather than letting her web of schemes play out to societal collapse or such

If there is anything we can glean from later data points, it isn't really the construct bit that's important. It's the platinum. Somehow.

We have a character called Mattias? Why didn't anyone tell me?

Anyway:

Gruff: Act of Treason. While he currently isn't really capable of controlled magic, in a pinch and if he's lucky enough to have a couple of mana bonds going on, he could probably manage a spell to influence someone's allegiance for a short while.

Hrmm... let me think...

Under 99.44 percent of circumstances, I'd say that Beryl tops-out with an Inferno. But, with that being said, I have a real suspicion that, if she ever felt like there was nothing left for her to care about, a Worldfire might just be the outcome.

Jackie doesn't really have any magic to speak of. Her trump card is made out of lead. But, for the sake of argument, we could consider Hush-Hush in her place, who can produce Upheaval-quality results with the right amount of crystal.

There isn't much subtlety to Gale's trump card. She's bringing the Hurricane.

Tryst is probably something like Drain Life -- she's the one I have the hardest time with, actually. There might be a better answer, but it's not coming to me at present.

Daneera seems like she would go for a good, old-fashioned Overrun and let her forest friends do the trick. If she doesn't have any creatures at that moment, she'd probably go to Dramatic Entrance.

Denner definitely goes to Phantasmal Image when he's in trouble.

For The Dual-Walkers, I was going to find the most powerful spell I could and say that one. Unfortunately, Yore-Tiller Nephilim seems to be the only one. So...Wrath of God?

When in trouble, Penelophine probably just whips out her Siren Song Lyre...

Cyrryc Adda would probably just drop a Serpent Generator on the ground and see what happens.

On a similar note, something tells me Lady Nasina would use a Decimator Web.

I'll do more at some point, but I'm tired now...

From what I know of the Dual-Walkers, Cruel Ultimatum feels like the best fit, despite lacking the white.
Otherwise, it might be best to find something different for each of them.

is there a dossier somewhere on Alessa? I'm curious 'cause I was thinking about this for the planeswalkers in the Ossia storyline (at least the ones that belong to me) and for Vella Voss I came up with probably warp world or scrambleverse, so I'm curious how similar the characters actually are.

If we're asking about dossiers then could I be directed to info on Dantalion. I have tenative plans for something that might be too similar to it, and I can't find anything n the archive or on the wiki to confirm or deny my suspicions.

PM me as much as you'd like. I can answer questions about Dantalion and arcs present and future.

I never have really gotten around to writing a proper dossier for Alessa, but she does have two relevant stories to your query.
Gifts covers her origins while Live for the Moment is an introduction to her abilities and character. Gifts is... probably a lot better than Live for the Moment, but LftM was an introductory piece, so what can you really expect?

Okay, here are several more. Man, I have a lot of characters...

Blink is just inclined to murder her problems away. You could argue that because of her methods, they are murderous betrayals.

Donagut, conversely, will just default to his murder investigation.

Although we haven't seen her do so yet, I imagine Ethal would summon the School if she needed help.

Jade, as a dreamwalker, would probably utilize Dream Fracture or, more darkly, Dream Leash.

Kahr-ret-Taris would look to enchant himself with Fists of the Demigod and Runes of the Deus at the same time...

Although he wouldn't like it, Lukas Harran would go to his Demonic Rising.

It stands to reason that Morgezka Shanak would simply bathe you in dragonfire.

For Raiker Venn, I really wanted to say Alter Reality, but the card is nowhere near as cool as the title, and does not reflect the Poet's abilities. Deny Reality is closer, I would say.

Sundar Elarion would self-cast Lightning Prowess on himself.

And Antine? I think he just wants a hug.

I haven't had a chance to explore Tey'von's skill set in either of the stories I've written about him, although I have made decisions about it. Basically, his commitment to non-aggression means that he will never strike first, and will do everything in his power to avoid lethal force. If someone attacks him with such force, he might find himself forced to use either Mirror Strike or Deflection, depending on the kind of attack.

So, coming back to this, I realized that Warp World isn't really that accurate for Alessa. Rather, Possibility Storm is her spell of choice when things are out of control.

But I have kind of settled on what the heaviest hitter in our roster does when the chickens have come to roost.

Zhiran:
Tower of Murmurs
Tower of Fortunes
Tower of Champions
Tower of Eons
Tower of Calamities

For the record, this is another example of something in my mind that has a reprint divorced of setting. There is a plane out there somewhere that has these towers on them that tell a different story.