Manaphy has been dropped back into OU, so lets talk about our newest friend
What can manaphy offer?
A number of things really:
manaphy is a mono water typing, which overall is an excellent typing
with its 1.2lb weight manaphy takes virtually zero damage from grass knot, making its mono-water typing even better
Base 100 in all stats make it versitile, though its movepool is not so accommodating to all sets.
hydration is the big one. A unique ability that lets manaphy heal all status in raid, letting it awaken from rest immediately, and preventing status from shutting down a sweeper
But manaphy does have some cons:
as said before its movepool overall is quite shallow. It does not have the amount of sets as say celebi, another 600 bst poke, does.
100 speed is good, but not quite enough to be a truely threatening sweeper. Numererous thunderbolt carrying attackers can all outspeed it.
Hydration can sometimes come back to hurt you- well timed switches of weather changers can cripple manaphy
Lets talk about some sets next. The most well known is probably the tail glow sweeper from the ubers anylsis, looking something like
4 hp/252 spa/252 spe
life orb
tail glow
surf
ice beam
energy ball/rain dance/rest/hp electric
The big difference between OU and ubers is kyogre. Without the permenent rain manaphy either has to carry rain dance, losing coverage, or rely on other rain dancers. Alternatively it can forgoe rain and go as is. I used this set primarily in suspect, and without rain it still is at risk of getting walled by blissey. The main risk here is rain only lasts so long in OU, and manaphy can take some time to set up for a proper sweep, eating up precious rain turns. Energy ball is generally superior to grass knot, for hitting opposing manaphys and vaporeon switch ins.
Crophy: A set not run in ubers, this works almost the exact same way as suicune does.
252 hp/252 def?(I'm not good at evs)
leftovers
rest
rain dance
calm mind
surf
Manaphy can switch in, set up rain dance, and begin to calm mind up. Rest allows it to instantly heal in the rain. Risks on this set are naturally vaporeon completely walling it, and weather changers. A well timed weather switch can leave manaphy sleeping for 2 turns. Trick also ruins you, so be wary. An alternate to this set is switching calm mind with toxic, for a stall playstyle
Something I concocted on my own:
252 atk/252 spe/4 hp
life orb/damp rock
waterfall
return/U-turn
rain dance
rest
This is manaphy acting as either a physical attacker(return/life orb), or rain dance support(damp rock/U-turn). Return+waterfall offers almost universally unresisted coverage, only empoleon resists both. Rest and rain dance allow you to boost waterfall damage, and recover LO recoil/status.
Alternitively a damp rock and U-turn can be used, letting manaphy work as a primary rain dancer for rain based teams.
Who do you want on your tem with manaphy?
The obvious ones for rain dancers are swift swimmers. Kingra, ludicolo, kabutops and the like are all well known rain dance teamates, and manaphy can provide them with rain, or take advantage of rain support from others.
With grass knot being the most common grass move, manaphy's main fear is electric attacks. This makes ground types an obvious choice. Swampert grounds electric, and can use rain boosted surfs and waterfalls. Manaphy intercepts grass knots without worry.
Suicune might seem to be less useful with crophy around, but the two can work together in the same way that salamence and dragonite can, creating a double calm-mind rest after a counter has been sufficiently weakened or removed.
What can you do against manaphy?
Manaphy being voted ou must mean it has counters right? well here are some options:
Blissey can come in sometimes on the tail glow sweeper, but a +6 surf in the rain will 2hko her
Latias resists water, and can fire off super effective thunderbolts while having a speed advantage over manaphy
Abomasnow also resists water, and grass, and is neutral to ice. A well timed swich can ruin a manaphy hydration. After that abomasnow can threaten the sleeping manaphys with stab wood hammers, or set up leech seed and try to stall out manaphy.
Ludicolo sports a 4x resist to water, and is neutral to grass and ice. It also benefits from rain both by outspeeding manaphy with swift swim, or being able to heal more with rain dish. It can then threaten to stall out manaphy's rain with leech seed, protect, and rain dish, or simply outspeed and use stab grass attacks.
Jolteon and raikou both outspeed manaphy, and can hit hard with stab electric attacks.
Vaporeon can completely wall manaphys without energy ball.
Rain dancers in general can threaten manaphy with its own weapon. Kingra has a 4x water resist and has powerful dragon attacks to use against manaphy.
Tyranitar and hippowdon are weak to water, but can also ruin hydration. Use with care.
Here's to hoping manaphy can shake up the metagame a little, and find more use than it did in ubers. Comments/corrections/improvements?

What can manaphy offer?
A number of things really:
manaphy is a mono water typing, which overall is an excellent typing
with its 1.2lb weight manaphy takes virtually zero damage from grass knot, making its mono-water typing even better
Base 100 in all stats make it versitile, though its movepool is not so accommodating to all sets.
hydration is the big one. A unique ability that lets manaphy heal all status in raid, letting it awaken from rest immediately, and preventing status from shutting down a sweeper
But manaphy does have some cons:
as said before its movepool overall is quite shallow. It does not have the amount of sets as say celebi, another 600 bst poke, does.
100 speed is good, but not quite enough to be a truely threatening sweeper. Numererous thunderbolt carrying attackers can all outspeed it.
Hydration can sometimes come back to hurt you- well timed switches of weather changers can cripple manaphy
Lets talk about some sets next. The most well known is probably the tail glow sweeper from the ubers anylsis, looking something like
4 hp/252 spa/252 spe
life orb
tail glow
surf
ice beam
energy ball/rain dance/rest/hp electric
The big difference between OU and ubers is kyogre. Without the permenent rain manaphy either has to carry rain dance, losing coverage, or rely on other rain dancers. Alternatively it can forgoe rain and go as is. I used this set primarily in suspect, and without rain it still is at risk of getting walled by blissey. The main risk here is rain only lasts so long in OU, and manaphy can take some time to set up for a proper sweep, eating up precious rain turns. Energy ball is generally superior to grass knot, for hitting opposing manaphys and vaporeon switch ins.
Crophy: A set not run in ubers, this works almost the exact same way as suicune does.
252 hp/252 def?(I'm not good at evs)
leftovers
rest
rain dance
calm mind
surf
Manaphy can switch in, set up rain dance, and begin to calm mind up. Rest allows it to instantly heal in the rain. Risks on this set are naturally vaporeon completely walling it, and weather changers. A well timed weather switch can leave manaphy sleeping for 2 turns. Trick also ruins you, so be wary. An alternate to this set is switching calm mind with toxic, for a stall playstyle
Something I concocted on my own:
252 atk/252 spe/4 hp
life orb/damp rock
waterfall
return/U-turn
rain dance
rest
This is manaphy acting as either a physical attacker(return/life orb), or rain dance support(damp rock/U-turn). Return+waterfall offers almost universally unresisted coverage, only empoleon resists both. Rest and rain dance allow you to boost waterfall damage, and recover LO recoil/status.
Alternitively a damp rock and U-turn can be used, letting manaphy work as a primary rain dancer for rain based teams.
Who do you want on your tem with manaphy?
The obvious ones for rain dancers are swift swimmers. Kingra, ludicolo, kabutops and the like are all well known rain dance teamates, and manaphy can provide them with rain, or take advantage of rain support from others.
With grass knot being the most common grass move, manaphy's main fear is electric attacks. This makes ground types an obvious choice. Swampert grounds electric, and can use rain boosted surfs and waterfalls. Manaphy intercepts grass knots without worry.
Suicune might seem to be less useful with crophy around, but the two can work together in the same way that salamence and dragonite can, creating a double calm-mind rest after a counter has been sufficiently weakened or removed.
What can you do against manaphy?
Manaphy being voted ou must mean it has counters right? well here are some options:
Blissey can come in sometimes on the tail glow sweeper, but a +6 surf in the rain will 2hko her
Latias resists water, and can fire off super effective thunderbolts while having a speed advantage over manaphy
Abomasnow also resists water, and grass, and is neutral to ice. A well timed swich can ruin a manaphy hydration. After that abomasnow can threaten the sleeping manaphys with stab wood hammers, or set up leech seed and try to stall out manaphy.
Ludicolo sports a 4x resist to water, and is neutral to grass and ice. It also benefits from rain both by outspeeding manaphy with swift swim, or being able to heal more with rain dish. It can then threaten to stall out manaphy's rain with leech seed, protect, and rain dish, or simply outspeed and use stab grass attacks.
Jolteon and raikou both outspeed manaphy, and can hit hard with stab electric attacks.
Vaporeon can completely wall manaphys without energy ball.
Rain dancers in general can threaten manaphy with its own weapon. Kingra has a 4x water resist and has powerful dragon attacks to use against manaphy.
Tyranitar and hippowdon are weak to water, but can also ruin hydration. Use with care.
Here's to hoping manaphy can shake up the metagame a little, and find more use than it did in ubers. Comments/corrections/improvements?