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
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.
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.
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.
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.
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.
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 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).