The problem with recommendation questions is that everyone can put down their favorite candidate and it just becomes a popularity contest.
On the other hand, a comprehensive list doesn’t have those, so here’s an attempt at one:
Planescape, or simply “the multiverse” in 5e terminology, is the campaign setting that covers the planes beyond the Material Plane that most campaign settings focus on. As the Material Plane is embedded within the multiverse, it’s a kind of “over-setting” even for other campaign settings. The multiverse consists of
- the Material Plane,
- the three or four Transitive Planes
- Astral, Ethereal, and Shadow,
- and in 5e, the Feywild,
- six Inner Planes of elements and energy, plus twelve border regions which may be planes in their own right
- Elemental Planes of Air, Earth, Fire, and Water,
- the Positive Energy Plane and the Negative Energy Plane,
- Paraelemental border regions between pairs of elemental planes: Magma (Fire+Earth), Ooze (Earth+Water), Ice (Water+Air), and Smoke (Air+Fire)
- Quasielemental border regions between an elemental plane and an energy plane: Radiance, Mineral, Steam, and Lightning for the Positive side, Ash, Dust, Salt, and Vacuum for the Negative side
- the seventeen Outer Planes of alignment and belief
- the true-neutral Outlands, plus
- a ring of sixteen planes around it, eight for the eight non-true-neutral alignments alternating with eight others that are halfway between their neighbors.
This cosmology is often referred to as the Great Wheel, since the emphasis is on the ring of Outer Planes.
Acheron is the Lawful-by-Lawful-Evil Outer Plane, between LN Mechanus and LE Baator (the Nine Hells). It’s described as a series of floating cubes, where gravity always points into an edge, and upon which an endless war is waged. The cubes move randomly and even collide at times, and various nasty weather accompanies the misery of the trenches.
Elemental Plane of Air
As you might guess, the elemental plane of Air consists mostly of Air. Gravity is subjective here, and “bubbles” of impurities can be found—vast spheres of water or other liquids, floating chunks of earth, and so on.
Gehenna is the Evil-by-Lawful-Evil Outer Plane, between LE Baator and NE Hades, and it consists of a series of massive mountains floating in an endless void. There are no natural surfaces that are less steep than 45 degrees, and the entire plane basically exists to drag everything down. You can carve out a place for yourself, but it is always precarious.
Paraelemental Plane of Smoke
Smoke, as the paraelemental plane between Fire and Air, is rather inhospitable (though perhaps not as much as Fire’s pure relentless heat), but there are floating chunks of charred land found within the choking gases.
The “other” Multiverse
Wizards of the Coast, of course, also publishes Magic: The Gathering, which has its own “multiverse,” and consists of numerous planes without any particular connection to one another (and, for the most part, only the mighty planeswalkers can travel between them). Recently, they have begun publishing some Magic planes as D&D campaign settings. At least one of them applies here:
Plane Shift: Zendikar describes Zendikar, a plane of incomparable wilds and energy, where the magic infused into the very land has much of it pull itself up out of the crust, and float freely above the world. The energy of the plane has been a substantial draw to planeswalkers and other, even more dangerous interlopers.