So...getting to one I've had in the back of my mind for a while...
Manaphy
What makes Manaphy good in the Uber metagame? Kyogre tends to cause constant rain, and then Manaphy heals status every turn, which, among other things, allows it to use Rest without spending any turns sleeping. This is fairly tough to crack in ubers, let alone Standard. Fortunately, Kyogre is not in standard. Kyogre will never be in Standard. So...Rain Dance lasts the usual 5 turns (or 8 with item), and you can probably spend those turns on something better than stalling and using rest. And that's assuming the opponent doesn't have an auto-hail or auto-sandstorm pokemon...which are common enough. In fact, as noted Manaphy has been tested before, and looking at Manaphy's move list in the experimental pool...most people aren't even using Rest at all on Manaphy.
SpeedManaphy loses to 9 OU pokemon, ties with 6 OU pokemon, and beats 35 OU pokemon. Definitely above average, but people already have the 100 speed number in their crosshairs, so they will pick speed numbers to put them 1 above certain 100 speed setups. Even allowing for that, it's 70th percentile or so.
Offence100 attack/special (compared to quite a few prominent pokemon with 130). Only gets STAB on water, but water's relevant (nails Heatran, nails Tyranitar). More specifically, water hits three relevant weaknesses (Rock, Ground, Fire) and is only resisted by three types (Water, Grass, Dragon), which, granted, are relevant types. (It's also worth mentioning that water's neutral against Steel).
Movepool is fairly varied, moreso in special. In Special has 90+ power attacks in Water, Psychic, Ice, Grass (...inconsistently; silly Grass Knot), and 75+ attacks in Ghost, and Bug. But it lacks the "120 with acceptable drawbacks" moves (including Hydro Pump) not to mention the "140 with acceptable drawbacks" moves (like Overheat and Draco Meteor). And...notably for OU, according to Shoddy statistics, the 10-most-used pokemon all frequently carry one of Fire, Earth, or Fighting (i.e. super effective against Steel). Except Latias (still has HP Fire on rare occasions). Manaphy gets no advantage for using Hidden Power, though--STAB Surf does as much as 70 power Earth/Fire/Fight to Steels. On a different note, naturally if you do use rain it will further boost the power of Surf. Notably, Rain+STAB = 2.25x, better than hitting a 2x weakness.
Manaphy's physical movepool is a lot less varied. It has Waterfall (good physical water move), U-Turn (some arguments for being the best move in the game--I've seen good players use Baton Pass without any stat raising move in their setup just so that they could get extra information on switch ins. Take that and add 70 Bug damage), and...some Normal options--Return for 102 power is probably the best. But...regardless, we're probably not going to see a dedicated physical Manaphy any time soon; a Special Manaphy with U-Turn, sure, but....
Stat boosters--I'm going to come back to this; I want to lay the groundwork first. Suffice it to say...Tail Glow.
DefenceManaphy's one of those pokemon with 100 in every stat. What does that actually mean? Fairly solid, though not godly durability (by the previous measure, a Manaphy with defencive EVs takes
21.0% damage--which is to say, only 8% less durable than Deoxys-D >_>). With the max HP EVs, reaches 101 HP Substitutes. The status recovery in the rain is pretty cool, but honestly, if you're going to have one of your pokemon use a move so that another pokemon can recover status, there's Heal Bell. The Natural Cure ability on, for instance, Celebi will usually be better. In fact, in general I'm not too worried about pokemon who can be described as "only over the top on Rain Dance teams"--Rain Dance doesn't seem to be pushing the boundary on "too good" right now...and even if it was, something tells me mono-type teams that require setup aren't going to dominate.
Water on defence is pretty good--four relevant resistances (Fire, Water, Ice, Steel) and two weaknesses (Grass, Electric)--both from attack types with about twice as many resistances as weaknesses. Also, that grass weakness? Grass Knot is popular, and is only a 20 power move against Manaphy. So...pretty tanky resist-wise...although the competition in the "100 in all stats" category includes Jirachi, who also has only two weaknesses...and 9 resistances.
Manaphy against the battlefield is...neutral to everything. Normal damage from Stealth Rock. Can be poisoned, burned, paralyzed, hit by hail, hit by sandstorm, hit by spikes, hit by toxic spikes. I guess being a primarily special attacker it cares less about Burn than most. To be fair, this applies to Celebi and Shaymin too...although they cure status more easily. Jirachi's tankier here, though (can't be hurt by sandstorm, takes less from Stealth Rocks, and can't be poisoned).
So...outside of getting really meta (a la "everyone prepares for steel!!!"), durability goes Jirachi > Manaphy > Celebi.
SupportManaphy lacks healing. Rest is basically its only option. (This is compared to Recover on Celebi, and Wish on Jirachi). Past that, Manaphy has some decent-looking support options. Heal Bell (cure party status). Sleep Talk (could be paired with Rest). Dual Screens (i.e. Reflect and Light Screen). Safeguard. And it does learn Rain Dance in case you want to alternate that and Rest.
Notably, Jirachi learns all of these except Heal Bell (Celebi learns all of these). And...at least in the suspect latter, most of these were pretty rare (since if you want support, Jirachi does those AND Stealth Rock).
The most-used one, Rest, was used on about 20% of Manaphys, so some people try to make it work, but it's far from the consensus setup (unlike in ubers where its usage is like...95%). I should also note, full healing is good, but if your opponent's dealing over 50% then you are heal-locked...and in Pokemon it's blind heal-locked (you don't actually know before they attack whether they will attack or do something else). And naturally, if someone predicts a Rest and switches in Tyranitar or Abomasnow, then you're out cold for three rounds.
One more comment on the Hydration+Rest+Rain Dance Combo, Manaphy's not the only one that can use it; there are others, and they're all in the "Never Used" tier. Granted, their BST is more middle of UU unlike Manaphy (whose BST would be tied for best in tier), but I'm going to theorize that if this move combination made you way better than your stats suggest, then presumably these other Hydration pokemon could keep their head above water in UU. And if this move combination does not make you way better than your stats suggest...well Jirachi has a lot of things that ought to make it look better than its stats suggest (Wish, Stealth Rocks, Steel Typing, U-Turn) and isn't uber. So...if something's going to keep Manaphy banned, that thing probably won't be Rest+Hydration.
Tail GlowSo...remember that thing earlier that I skipped over? Manaphy's stat-upping moves? Yeah, so uhh, Manaphy learns Calm Mind, but it's pretty rarely used. Perhaps because Jirachi learns it too. But more likely because Manaphy gets freaking
Tail Glow. For those who don't know it, Tail Glow is Nasty Plot is Swords Dance for special attack.
So...who among the 600 BST group can we compare Tail Glow to? Oooh, Garchomp has a +2...wait, he's banned. Umm...Shaymin and Celebi have +2s in their less favoured stat that they don't have good moves for.... To be perfectly egalitarian in this comparison, I'd like to go for a special attacker like Heatran...wait, he doesn't have a stat-up at all. So...who else uses Tail Glow? Some 400 BST bug with worse stats than Ponyta. Err...okay, who else learns Nasty Plot? Well...notably Azelf, a pokemon that beats Manaphy in speed and special attack.
So...Azelf hits harder and faster. But...arguably being tanky is important for a sweeper, just to make sure that the priority moves don't stop you. And Azelf scores low on Tanky (30.8% using the same defensive measure). Well...Togekiss...if we ignore the attack stat on both Togekiss and Manaphy (which, frankly, neither of them are using as Nasty Plot sweepers) then their stats look similarly good; Togekiss is a bit slower, but hits a bit harder, and is similarly tanky. Oh, and Togekiss also gets two support moves that pair well with Nasty Plot: Baton Pass and Roost (Roost being arguably better than Rest).
It is noteworthy that rain dance teams get to give Manaphy an extra +50% before it uses Tail Glow (although this only boosts Surf. Against Salamence or Latias or Celebi, Ice Beam's still better than Surf even in the rain, so it's not a straight +50% damage to all attacks).
---------------
All in all, Manaphy's really feeling like "not uber" to me. It just tends to be boxed in on two sides. Azelf is the faster Nasty Plot sweeper; Togekiss is the slower Nasty Plot sweeper. Celebi is better at shrugging off status; Jirachi is tankier. (And outside of Tail Glow they both learn better support moves). Speaking of Celebi, I'm sure Celebi would love to have a water type to have an advantage over. And as far as rain dance teammates go, I'm going to say Swift Swim impresses me a lot more than Hydration.
The other reason you might ban pokemon is just metagame reasons. I mean, I can see arguments that, even if individually they're all balanced, unbanning multiple Deoxys and Latios and Latias would be silly, as it would overrun the metagame with psychic pokemon. That said, water, while present in the metagame, does not threaten to overrun much.
You could also argue that too many pokemon with 100 in all stats removes variety, but in that case you'd want to do some head-to-head comparisons between Jirachi/Manaphy/Celebi (and assuming they come out even, I might swap Jirachi into Ubers and Manaphy out anyhow--Jirachi leaving means one less 600 BST Steel type, and one less Stealth Rocks user).
