Black hole.

“To all Admirers of Curious Dimensions, Exotic Formations and Giant Celestial Spheres, I invite you on a Joyride through the Cosmos. Along the Majestic Nature and its Overwhelming Splendor. In terms of size, we can safely say that these Superlative Objects will Transport you across the Universe, past the Surprisingly Large Voids, through the Oceans, to the mysterious Yellowstone, and ending at the Black Hole, the ultimate frontier of the cosmos!”

1. Atmosphere
2. Biosphere
3. Continent
4. Dervish (type of galaxy that can be very large)
5. Ecosystem
6. Firmament (heaven or expanse)
7. galaxy
8. Universe
9. Ice cap (for example, that of the North or South Pole)
10. Jovian planet (large gas giants such as Jupiter and Saturn)
11. Cosmos
12. Landmass
13. Milky Way (our own galaxy)
14. Nebulous (large cloud of gas and dust in space)
15. Ocean
16. Planet
17. Quasar
18. Room
19. Supercluster (a group of many galaxies)
20. Tectonic plate
21. Universe
22. Voids (Immense spaces between the filaments (the largest structures in the Universe))
23. World Ocean (the totality of all oceans on Earth)
24. X (unfortunately, I couldn't find a large object that starts with an X)
25. Yellowstone Caldera
26. Black hole