What would you consider to be the most urgent thing that needs to be tested?

I haven't had much experience with Manaphy, but I'd imagine it to take quite a bit of time to set up the Rain Dance, Tail Glow, and Rest. With that it only gives one attacking move that I think would severely limit it. During such time it would be easily Tricked a Scarf, Taunted, etc. I usually don't give my opponent even two turns to set up before I try to do something crippling to them.

Then again I could see it surpassing many OU threats, but I think some testing would really clear things up.

Edit: Let me remind everyone that if we are to test something you can prove someone right or wrong on the battlefield instead of simply theorymoning. Its one thing to make a guess and another to test it out, so please try to keep an open mind to things.

Im not even going to waste time w/Latios/Latias even w/o Soul Dew they have incredibly ridicoulous stats/movepools.

I dont see the big threat of Heart Swap, who boosts Special Attack, maybe if Manaphy was a physically, based pokemon it could be a decent Scizor killer, but it Physical move pool is shallow mostly non-existent, holding Waterfall and Return, U-Turn but that goes against the point of a Heart Swap set. Heart Swap might good against a few nasty plotters or a Decent Yanmega counter but other than that it a gimmicky, utility move-set.

-Waterfall
-Return
-Heart Swap
-U-Turn/Natural Gift (pathetic)

A special Heart Swapper might be a bit more usable, but as said before SAtk boosters are rare, it would be the best Suicune/Celebi Calm Mind counter, but it'd just draw out a Blissey, Kingdra to set-up in its face.

-Surf
-Ice Beam
-Grass Knot
-Heart Swap

nothing i think all this "testing" is just causing too many metagame shifts to leave a enjoyable game.

the only thing i really would even vote for if forced to is a test of a SRless metagame

I dont think my current status allows me to vote, so I'll just say that I would vote for SR removal. (I'd also like Skymin to go)

without it, i think someone here says in their signature that they dont use stealth rock because it doesnt take any skill to set up, and they're right

A lot of people rely on SR to make those 2HKOs turn into 1HKOs, and 3HKOs into 2HKOs, and so on. Hell it even prevents some would be counters from ever seeing the light of day.

For example, Moltres and Charizard, both have potential to be monster threats almost never get used because they lose 50% right off the bat.
The only reason why i see SR being actually being fair and clever is to prevent Ninjask and Shedinja from completely dominating.
Another reason why to keep SR is because then you would need something like Forresstress on your team, who would otherwise be without a job, unless you use it to set up Entry Hazards, which by then arent we back where we started?

As to the other choices, I reject all of them.

1. Garchomp was overcentralizing the metagame. It essentially took 2-3 pokemon down with it before it can be killed, because you cant beat it one on one. If Garchomp switches into something like Tyranitar or Salamence, any sensible user would switch out because Garchomp can KO both, creating a free Swords Dance. Then it took something that can take 1 +2 Outrage, often Cresselia, to activate the first Yache Berry. Cresselia's definitely dead on the second Outrage if not the first. Finally, you had to have something faster, to revenge kill, say Weavile, a Scarfer, HP Ice Gengar or something. But then Garchomp can just switch out, come back and do some more damage.

2. Latias/Latios in OU is a bad idea. It may be true that without Soul Dew, it isnt Uber but that doesnt make it OU. Latios has the same SpAtk as Gengar, as well as 110 Speed. Its not physically frail either (it just seemed that way in ubers because of how high the offensive stats the ubers ran) Whoever said it has a bare movepool isnt looking hard enough. It will arguably be the best BoltBeamer, as well as STAB DrgPulse and Psychic. It also has Shadow Ball. It can carry HP Fire for steels, hell it could even run support sets with Safeguard and Wish(Latias)

3. Mew may suck in Ubers(not true btw, its outclassed as a sweeper, yes, but it can run support sets,) It's most valuable trait is Baton Pass, as it can any +2 Stat Up, even Barrier. In OU, Nasty Plot/Swords Dance with the amount of options available to it, along with its respectable bulk will make it hard to contend with. Manaphy is the same situation, all its losing is Hydration really, and if Manaphy finds itself on the same team as a Kingdra would already demand rain support, it would be unstoppable

One on one? I can think of tons of pokemon that beat it one on one. Gliscor, Cresselia, Hippowdon, Weavile, Mamoswine...

I thought that any sensible user would force the Garchomp out first with Gengar before attacking with the bulky wall with ice attacks, and it will either flee at the first sign of Gengar or lock itself into Outrage, making it easy prey for the Cresselia of Weavile. Or perhaps simply attack with aforementioned Dragon, safe in the knowledge that the Garchomp will be afraid to attack for fear of being revenge-killed, or worse, set up on, eg. Swords Dance Weavile.

Anyway, surely the same is true of many other pokemon? Breloom, Scizor, Lucario, Gallade, Weavile, Ursaring, Pinsir, to name but a few? Aren't they bound to crush at least two pokemon before collapsing in a heap, after a boost?

These are not counters. They switch in as Garchomp SD's and either attack him activating his Yache berry and get OHKO'd (weavile, mamoswine) or attack him and survive, but fail to outrun him and get 2HKO'd (Gliscor, Cresselia, Hippowdon).

I would also like to hear how a Gengar is supposed to force out a garchomp.

These will all be tested in the future; if you would look at the Order of Operations thread in Policy Review, we have set out a schedule. Also, the Arceus issue is not an issue of power, but of legality with game mechanics.