Which animal would you domesticate if you could pick one?

Foxes. They're like a cross between cats and dogs, with their cute fluffy tails.
- You should listen to and smell a fox before embarking on that kind of project.
- Russians inadvertantly did that, when trying to rear blue foxes for fur. Selecting for tameness altered the coat.

I'd take a unicorn or dragon.

Ocelot
- BABOU!!
  
  Lana! He remembers me!
- How do you titilate an ocelot?

Wasps; I'm going to be a supervillain now.
Edit: Scratch that. Termites have better crime potential, if that's the idea I'm going with.
Even if not, a large quadruped is the wrong answer when we already have dogs, horses and elephants. Something flying, digging, tiny or aquatic is where we should all be thinking.

Domesticated microraptor. It would be cool as hell, and also imply that we've somehow resurrected a dead species.
- Get a chicken. They still know they're dinossaurs.

I mean, obviously a lion.

Sparrows.
- Sparrows are already semi domesticated due to living near human settlements basically since we started farming.
  
  Well, they're not domesticated enough to let me pet them unless I catch a chick which fell off a nest.

Probably red pandas. I'm still a bit surprised they are wild animals, they seem too silly to be like...doing their own thing out there.
- Is this a rabbit hole I want to go down?
- I too want to domesticate a Firefox

A bear. They are just adorable and silly and cute until they eat you completely for no reason.
- A bear walks into a bar and says "I'll have a ................... beer." The bartender asks: "Why the big pause?" The bear responds: "I don't know, I was born with them."
  (Pic in OP relevant. Even bear cubs have big paws.)
- It's nice to be on even terms.

Pallas's cat

Penguin, I want to see a penguin wearing thigh highs and an arch linux shirt :3
- those would be the shortest thigh highs ever

Zebras. It's just a bug that we can't, right? It's just a stripey horse!
- Zebras are extremely aggressive and violent
  
  I'm assuming the hypothetical domestication process proposed by OP is magical though.
  
  I've read somewhere some have been successfully (more or less) tamed.

Quokka.
I need that type of positivity in my life.

Crow/raven!
- I am trying to befriend crows right now so I agree with this haha
  
  How’s it going? What are you doing to try?
  This is on my wish list of things to do, but I always forget to start.

Tall horse 🦒
- I always thought of them as necky camels more than horses
- Stupid long horse, but yeah that would be a sick
  
  Get me a miniature giraffe and a miniature elephant and I'm good.

Assuming their domesticated enough to listen to my commands and ignore their pray drive, on like a magical level, Tiger. Hands down. I would go to the Renaissance Fair with them and commission a blacksmith to forge armor for them. I would ride it into battle whenever necessary.
Edit: I was also assuming no one else has a magically domestic animal. If everyone in this thread also has one, I'll choose Rhinoceros
- So, essentially, you chose violence, be it through He-Man (Battle Cat) or the Teenage Mutant Turtles (Rocksteady).
  
  I just want to be ready, in case everyone else chooses violence.
  If, all of a sudden, there were a handful of people that had a magical connection with an otherwise dangerous animal; I imagine, either those people would fight each other or team up against someone with a robot army. Either way, a tiger would have a lot of weak points, even with armor.
- With those conditions I would pick a bear/panda or platypus respectively.

Fennec fox or fruit bat. Perhaps raccoon, or maybe otter. Seals are neat too if there’s water. There’s a lot honestly
- This actually seems realistic, foxes have been domesticated in a short period of time, and fennecs are adorable
- Fennec fox
  just LOOK AT THESE THINGS
  
  ears

Tigers are just big cats.

I'm gonna have to go with a miniature elephant, maybe the size of a Labrador.
- Pygmy elephant.

Capybara
- x2 for the chillest little dudes
- Straight up, chill coconut,

WHAT THE FOX SAY?

Don't know quite how things would work out, but definitely domesticat the North American beaver because they're one of the coolest animals and you can't convince me otherwise.

Cat
- Cats domesticated humans. Mine watches me clean her litter box. It's clearly a power play.
  
  Exactly, mine sees me as a bed apparently.

the human one

Mountain Lion, for home defense. Hard to act tough when your buddy just got dragged up to the balcony by their broken neck and stairs weren't involved in getting him there.
- There's a Youtube channel run by a couple with a domesticated mountain lion.
  
  That is not domesticated. It has dual passive income, no housing costs, and a source of free meals that is more convenient than eating said source of meals. That thing will maul somebody if the arrangement is no longer convenient and aligned with it's interests.
  
  this is crazy

Fruit bats. Just no ebola, please.

Red Panda. I want to steal one from the zoo every time I go.

Pallas cats look very soft, I'd love to be able to cuddle one
- manul
  
  Manul!

Wolf, I call dog
- Tbh, a pet that's friendly and tameable like a dog but physically a wolf would be cool. I think wolves are very cute.
  
  The closest you can get is a malamute, assuming the wolf in question is like a grey wolf
  
  They are sweet creatures. But they need to run! And are social creatures like humans. They need to have families with them :)

Red fox would be great assuming domesticated would render them not smelly xD

One that's big enough to cuddle us. They also should get along fine in the increasingly small environments we are being shoved into. Having a newly domesticated animal that is limited to the ultra rich with giant estates is pointless, right? Bonus points if we can make it an environmental win as well.
Just about everything 'big' fails at being in an apartment though. Big cats, even the smaller ones among them, need more room. Same with bears, moose (cooool, dudes, am I right? Imagine lounging with a moose), elk, whales, dolphins... bah.
My idea, then? Giant birds. Big enough to lay on you when on the couch, but could be let loose to fly around while you're at work. It would take an incredible amount of domestication to get them to go and come back, but if we're talking hypotheticals and theoreticals and blue fantasy, I think giant birds would be the way to go. Take your pick of them. The steller's sea eagle and especially the phillipine eagle speak to me, but I wouldn't turn my nose up at a swan, a goshawk, or a red kite. Feed them well at home, pretend that during the domestication process we made the nesting area easy to clean, and imagine being able to set it loose on stupid fascist leaders.

Kind of surprised that I'm not seeing honey badger or wolverine or tasmanian devil.

Gorillas and monkeys. Just to watch them evolve. I think it would be nice to see them with glasses.
- Gorillas are intelligent enough to learn sign language, so if you could teach a group, nature, uh, nature would find a way
- Is this what you want?
  
  Monke
  
  I was thinking more like the tv show The Umbrella Academy but this is definitely worth a laugh! I don't know how to post a photo or I'd send you one back for reference. None the less if I see a monkey push up their glasses every so often as they slide down. I'd have to laugh.
- literally the plot of Conquest of the Planet of the Apes (1972).

Opossums.

Otters

Snow leopard

Parrots can be extremely intelligent, but are still dinosaurs.

Tiger!

Yeah, bears for sure. Little teacup-size pet bears.

Miniature gm giraffe, about waist height. Cantering about with its tiny blue tongue. I love that little freak.

A pet kangaroo would be cool
- a fluffy wallaby!

Having a pet sloth would be pretty cool
- It will be a long process.

Zebra. Many have tried and gotten killed in the process.
Alternatively, Triceratops.
EDIT because I brainfarted: I meant zebra. Donkeys are perfectly stable and friendly. Zebras are assholes.
- I once had to help an ass that had either been dumped or escaped and ended up at a park until the animal wrangling guy came. It was the sweetest creature I could ever imagine. All it wanted was to stand next to me and lightly push its head against my arm until I gave it pets and scratches. One of the best 30 minutes of my life.
  
  I made a zebra out of myself due to having a brainfart as I wrote my original comment. See edit.

I think a wombat would be cool to have around the place, assuming domestication allowed for some level of trainability.
One of my old uni lecturers was very keen on the idea of domesticating the spotted quoll. He reckoned they would make a great pet for those who would otherwise get a cat (a convincing argument actually).

Zebra or giraffe. Not to ride, just to go on walks with.

ears