6a: The Good
Ground Immunity Abilities:
Levitate ignores damage-dealing Ground-type moves, while also granting an immunity to Spikes and Toxic Spikes. Earth Eater grants an immunity to all Ground-type moves, restoring 25% health to the user if hit by one.
Levitate and Earth Eater are extremely similar, and most Pokemon running one can also viably run the other. Levitate is more useful on Pokemon prone to getting worn down by Spikes, while Earth Eater is preferred on Pokemon that don't much mind entry hazards and would prefer the 25% health buff. Ground immunity abilities are best utilized by Pokemon either weak to Ground or bulky neutral Pokemon looking to become a sudden check to threats like Great Tusk and Sandy Shocks.
Common Ground-immunity ability users:
Uncommon Ground-immunity ability users:
Niche Ground-immunity ability users:
Users you might consider: Ground-weak walls or generally bulky Pokemon that greatly appreciate an additional immunity.
Well-Baked Body:
Well-Baked Body grants an immunity to Fire-type moves and, when hit with one, raises the user's Defense by 2 stages.
Well-Baked Body is naturally very prominent thanks to the popularity of powerful Fire-type attackers like Cinderace, Moltres, and Iron Moth. Any Fire-weak or generally bulky Pokemon is able to take advantage of the additional immunity to switch in on both powerful attackers as well as stray Will-o-Wisps. This move also synergizes well with Body Press thanks to the +2 Defense boost.
Common Well-Baked Body users:
Uncommon Well-Baked Body users:
Niche Well-Baked Body users:
Users you might consider: Fire-weak walls or generally bulky Pokemon that greatly appreciate an additional immunity.
Electric-immunity Abilities:
Volt Absorb grants an immunity to Electric-type moves and, when hit with one, heals the user by 25% of it's HP. Motor Drive grants an immunity to Electric-type moves and grants a +1 Speed boost if hit by one, while Lightning Rod grants a +1 Special Attack boost.
Similar to Well-Baked Body, the prominence of powerful Electric-types like Zapdos leads to the rise of Electric immunity abilities to counter them. Volt Absorb is by far the most common, especially on bulky Pokemon and walls that heavily appreciate the health increase, but Motor Drive and Lighting Rod have their use cases on fast-paced offensive teams on threats like Manaphy and Gholdengo.
Common Electric-immunity ability users:
Uncommon Electric-immunity ability users:
Users you might consider: Bulky walls that appreciate an additional immunity for Volt Absorb, powerful offensive threats that appreciate a Speed/Special Attack boost for Motor Drive / Lightning Rod.
Water-immunity Abilities:
Water Absorb grants an immunity to Water-type moves and, when hit with one, heals the user by 25% of it's HP. Storm Drain grants an immunity to Electric-type moves and grants a +1 Special Attack boost if hit by one.
Water-immunity abilities are slightly more niche without the omnipresence of Walking Wake, but powerful Water-types like Ogerpon-Wellspring and offensive Manaphy give them a place in the metagame. Water Absorb is by far the most common, especially on bulky Pokemon and walls that heavily appreciate the health increase, but Storm Drain can see use on offensive threats that more greatly value the increased damage.
Common Water-immunity ability users:
Uncommon Water-immunity ability users:
Niche Water-immunity ability users:
Users you might consider: Water-weak walls or generally bulky Pokemon that greatly appreciate an additional immunity or Special Attack boost.
Bulletproof:
Bulletproof grants an immunity to certain "bullet" moves, a list of which can be found
here.
Bulletproof stands out from it's contemporaries by not granting an immunity to not just a certain type of move, but rather moves from several types. Moves like Shadow Ball, Focus Blast, Weather Ball and Energy Ball are all extremely widespread and are all blocked by Bulletproof, alongside slightly rarer but still relevant moves like Sludge Bomb, Pyro Ball, Aura Sphere and Zap Cannon. Bulletproof can be used by nearly any Pokemon looking to check a wide variety of special attackers, especially primal weather users dependent on Weather Ball for their STAB move.
Common Bulletproof users:
Uncommon Bulletproof users:
Users you might consider: Bulky Pokemon looking to act as a check to a wide variety of attackers.
6b: The Maybe?
Purifying Salt:
Purifying Salt grants an immunity to volatile status conditions and halves the damage of Ghost-type moves.
Purifying Salt is relatively rare, but it sees use on Pokemon who appreciate the resistance to Ghost-type moves but especially on physical attackers who use it as an immunity to Burn. It's usually a hard ability to justify, as while resisting Ghost-type moves is certainly valuable, a full immunity ability or an offensive amplification ability can often be preferable.
Uncommon Purifying Salt users:
Users you might consider: Bulky Pokemon who appreciate the additional resistance, or attackers that appreciate the immunity to
Soundproof:
Soundproof grants an immunity to certain "sound" moves, a list of which can be found
here.
Not too dissimilar from Bulletproof, Soundproof grants an immunity to several moves from several different types. Where Soundproof differs, though, is that sound moves affects relatively few and rarer moves. The only viable Pokemon who primarily attack with sound moves are Pixilate Scream Tail and Kommo-o, as well as Pecharunt with Parting Shot and RegenVest Primarina with Psychic Noise. While these are all common foes, it can be risky to dedicate an entire ability slot just to checking them. Despite this, Soundproof still sees rare use to turn near any Pokemon into a perfect counter to Scream Tail.
Uncommon Soundproof users:
Users you might consider: Bulky attackers on teams that
really hate Pixilate Scream Tail
6c: The Bad
There is essentially zero scenarios where Flash Fire is even a sidegrade to Well-Baked Body, as it is normally strictly worse. Similarly, Dry Skin has very few use cases over Water Absorb, as the Fire-type weakness is too notable to justify healing in rain.
Sap Sipper may seem appealing, especially on Swampert, but Grass-type moves are rare enough to make it a poor use of an ability slot, especially when Swampert gets so much more value out of RegenVest. Wind Rider is also a poor use of an ability slot, as wind moves are relatively rare and that ability slot is better spent on a guaranteed value ability like Regenerator.
Resistance abilities such as Thick Fat and Heatproof are strictly worse than either general defensive amplification abilities like Vessel of Ruin or their full immunity counterparts. There is nearly zero benefit to resisting a hit instead of absorbing it.
LordBox (Tier Leader)
Atha
Quantum Tesseract
Tranquility
Ivar57
DeepFriedMagikarp
Kinetic
Gimmicky
Slither Wing
Betticus IV
(+1) 
(+1)
(Vacuum Wave)
Regenerator, Well-Baked Body, Steelworker
Surge Surfer, Sharpness
Iron Valiant to High.
Glimmora
Kilowattrel