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. 

><sup>[Source](https://www.inverse.com/article/37759-thor-ragnarok-post-credits-scenes-thanos-infinity-war)</sup>

and...

>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.

><sup>[Source](http://www.indiewire.com/2017/10/thor-ragnarok-post-credits-scenes-what-they-are-1201887254/)</sup>

**Update** (2017-11-15)

Confirmed by Kevin Feige to [The Wrap](https://www.thewrap.com/thor-ragnarok-post-mid-credits-scene-explained-kevin-feige/)

> > “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.