Cresselia
I'd honestly want to see Cresselia added to this list. I've been toying around with a couple Cresselia teams lately, and so far this particular Pokemon has proven to be pretty effective at tanking and sweeping quite a lot of special attackers in the metagame, and even goes so far as to wall physical threats to a small degree as well.
This set is taken directly out of the team I've been using lately. This team has been enjoying a good amount of success on the ladder, with Cresselia winning many key fights in the game.
Great Wall (Cresselia) (F) @ Leftovers
Trait: Levitate
EVs: 252 Def / 96 SAtk / 160 Spd (add 4 if carrying HP Fire, out of SpA)
Timid Nature (+Spd, -Atk)
- Psychic
- Ice Beam
- Moonlight/Hidden Power Fire/Energy Ball
- Calm Mind
Timid Nature and 160 EV's in speed bring it up to 270, just enough to outspeed Adamant Dragonite, along with many other potent, non-choiced threats.
252 Defense brings it up to a solid 339. Along with a solid 384 HP, it's able to survive a plethora of physical attacks from a lot of the metagame, thus eliminating much of a need for Reflect, as this is a more offensively-inclined set.
The remaining 96 EV's go into Special Attack, so that it has at least some impact coming out of a +1 Calm Mind. (+1 SpA gives it 315 in that stat. Pretty nice, heh?) Quite often, I'm easily able to get to +2 or +3 before I start to sweep. Not only does this make Cresslia a monstrous special wall, but a potent sweeper as well.
Special Defense investments aren't necessarily needed, as 296 is an awesome number to start with. If I get a single Calm Mind in, this stat shoots up to an absurd 444, making it extremely hard to kill as is without a critical hit or a boosted attack from the opponent, if they're specially based. +2 makes it 592. These numbers serve to state how much of an impressive tank Cresselia can become if it is allowed to set up in front of anything.
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Psychic is the obligatory STAB, allowing it to attack a good portion of the metagame when the time comes. This one is pretty much obvious to have on Cresselia.
Ice Beam gives it coverage against most dragons in the metagame. As Cresselia can tank a large number of physical hits, it can at least fire off an Ice Beam most of the time before being forced to switch into something that resists Outrage, Dragon Claw & Company. It can do the very same thing to Tornadus, Thundurus, and other flying types that get in its way. (It does have problems with a Banded Lando's U-Turn, but if it's a different variant, it usually is able to OHKO with Ice Beam.)
Cresselia can set up in a specially-based dragon's face a lot of the time, as Calm Mind is an extremely potent walling and sweeping weapon that Cresselia possesses. If it can succeed in getting to +2, you're looking at 420 SpA, 592 SpD, and 384 HP that it can very easily use to attack anything carrying special (and even many physical) moves.
Moonlight is Cresselia's main and only source of reliable recovery outside of Leftovers. This lets Cresselia live for a long time indeed, although it's not seen a lot of use due to the very large presence of rain and sand in today's metagame. In sunlight, it is boosted to 66% recovery, allowing to to shrug off huge amounts of damage, while dishing out its own pain in return.
Hidden Power Fire may be used as an alternative, if Moonlight or Rest aren't to your tastes. It can abuse the sun for a 140 Base Power Fire move to OHKO most steels out there, barring Heatran. However, like Moonlight, it absolutely hates the rain, so you still need to watch out for the likes of Scizor and Genesect (especially the latter), if you've not had the opportunity to set up at least to +1/+2, and it's raining.
Energy Ball can also serve as an alternative attack that can discourage rain teams from making short work of you. However, you'll still have to deal with steels walking all over you.
Calm Mind's merits have been explained already, and there's no need at all to subsitute this move for any other, as it's key to Cresselia's longevity and power in many situations.
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Cresselia's checks and counters are somewhat obvious. Chansey & Blissey can wall it to infinity, even if she is running Calm Mind, as many of them like to carry Toxic.
Gastrodon also helps in wearing it down, but if Cresselia is set up, it can still remain there to kill Gastro (unless you chose to run Energy Ball instead of Moonlight/HP Fire).
Genesect can also wear her down with repeated U-Turns, if you don't carry Moonlight on you. A Banded version can even OHKO you.
Landorus is also a major threat, as its high-powered U-Turns have a chance of mowing this one down. Pretty much any competant Volt-Turn team can give her trouble, if you can't predict well enough to score enough kills to keep them at bay.
If you're carrying either a Moonlight or HP Fire variants of offensive Cresselia, rain teams will walk all over you the whole time the weather is set up, becuase your only viable attacking moves are now Psychic and Ice Beam.
Regardless, you'll still have trouble with Rain stall, especially in the likes of bulky waters, who will likely shrug off Energy Ball and recover with ease, unless you're able to set up in their faces (which is an iffy circumstance).
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Support for Cresselia is more or less vital. If you're carrying HP Fire or Moonlight, sun support is much more viable if you want to keep her attacking and recovering longer. It's not as vital if you're just using the CM + 3 attacks variant, as long as you've got Energy Ball to keep you protected against opposing waters.
Ninetales is of course obligatory for most variants for Cresselia, as he resists U-Turn. Both Ninetales and Cresselia work well together to give VoltTurn teams trouble, as many U-Turn users are either weak to, or do not resist Fire.
Thundurus-T is also a great option (in spite of it being likely a sun team), as its Volt Absorb ability really meshes together with the other two Pokemon, in the fact that it totally shuts down Volt Switch users (especially Rotom-W) in the sun. I've found that a Scarf variant is very effective, as you're able to fire off Volt Switches of your own to get Thundurus out of there in case you're wanting to use a simple hit-and-run technique.
Stuff that resist both fire and water are also good for this team, as well as things that tank Rock and Fighting moves, as they might be a pain for other support 'mons for Cresselia.
In the end, this thing doesn't need a lot of support to get off the ground and get rolling. All it needs is an opportunity to switch in on something it can set up on with impunity and get to sweeping.
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In conclusion, I would probably think this particular Pokemon would belong in the B-Tier, as it does need support, but can operate very well on its own if the circumstances call for it.