In a mid-credits scene at the end of Thor: Ragnarok we see
a large ship overshadowing and dwarfing Thor's escape ship.
Who does it belong to?
From my own answer over on Movies and TV
It has not been officially confirmed but the inference is that it's Thanos in his quest for the Infinity Stones...which will lead into the Avengers: Infinity War movie.
As Thor and Loki (Tom Hiddleston) relish in their victory, a giant spaceship with the very clear design and silhouette of Thanos looms overwhelmingly large, dwarfing Thor and Loki’s ship.
Our bet? Thanos, who has presumably gotten off his space rock in time to join the mayhem of “Avengers: Infinity War.”
The last time we saw the big guy, it was in a mid-credits scene in “Avengers: Age of Ultron,” one that saw the epic villain finally taking matters into his own hands (literally), donning the Infinity Gauntlet and vowing to “do it” himself. Part of getting the job done (the job being, of course, collecting all the Infinity Stones and using them for his nefarious aims) would presumably involve getting in a ship and going somewhere. If he found Thor along the way (plus Loki, who has already betrayed him), all the better.
Although Marvel has not released its first trailer for “Infinity War” yet, it did screen at Comic-Con, where reports held that it opens with a grievously wounded Thor getting picked up — floating, totally alone, in the middle of outer space — by the Guardians of the Galaxy. If Thanos attacked the spaceship holding Thor, Loki, and the rest of the Asgardians, potentially making off with it (and them), and leaving Thor to die in space, that certainly sounds like a hell of a jumping off point for “Infinity War.” While “Thor: Ragnarok” ends on an upbeat enough note, it’s clear that danger is around the corner, and there’s no greater danger than Thanos.
Yes, it is Thanos' ship. This is confirmed by director Taika Waititi in a recent interview
Q: OK, so the post-credits scene: Is that officially Thanos’ ship?
And that will be about all we know...
Q: Anything else you can tell me about it?
A: Nope. (Marvel is) so secretive! None of the directors are allowed to tell each other anything. No one reads each other’s scripts. I have no idea what Black Panther's (in theaters Feb. 16) about. I’m serious. I’ve never seen it. I have zero idea of the story line or anything.
The identity of the ship has been confirmed by studio head Kevin Feige. It's called the "Sanctuary II". Given that Sanctuary is the name of Thanos' asteroid base seen in the earlier films, we can reasonably assume that this is his flagship.
“We call it the Sanctuary II,” Feige told TheWrap.
For those who aren’t immediately familiar with that ship name, you’ll probably be quite alarmed to learn that’s the name of the flagship of Thanos (Josh Brolin), the MCU’s big bad who will finally take center stage in “Infinity War” after operating on the fringes of the story for most of these movies.
We have previously seen the original Sanctuary a couple times before — it’s the weird cosmic planet where Thanos sat on his throne in the first “Avengers” film as well as the first “Guardians of the Galaxy.”
Sanctuary II, being a space ship, is more mobile than some celestial body like Sanctuary, and it would make sense to find it on the move as we approach “Infinity War.” It’s not, at least in comics lore, a superweapon.
It's also explicitly stated in the film's script.
THOR: Probably not. But don’t worry, brother...I’ve got a feeling that everything’s going to work out.
[Then through the window, they see THE MASSIVE FORM OF SANCTUARY-2, THANOS’ WARSHIP approaching.]