Well, we started SP tier testing yesterday (will a topic be made for this later?) with testing Manaphy, Latias, Latios (both of these without Soul Dew), and Deoxys-LG (defense) in that tier.
So far, it looks like Manaphy is the biggest threat of those four in the tier, with the Tail Glow set wreaking havoc. The set being run the most is Tail Glow/Surf/Ice Beam/Grass Knot, without even having a need for Rest or Rain Dance, and it's been working fine. Think Mixape, but with Ice Beam instead of HP Ice. Blissey walls this set obviously, but I think it's only a matter of time until Sub/CM Manaphy pops up, or even Sub/Tail Glow. I know NYX has been trying a Rain Dance-based team starring Manaphy, but from what I've seen Manaphy hasn't set up too often; I honestly don't think Manaphy in Rain Dance will be a huge problem in SP, however, the threat is always there, especially since Manaphy can switch in after Rain Dance is used, Tail Glow up and survive a hit with its great defenses, and then OHKO nearly anything with Surf/Hydro Pump. There's always the possibility of Rain Dance/Tail Glow/Rest/Surf Manaphy, which has the potential to be extremely devastating, which certainly warrants further testing.
Deoxys-LG is also a fairly big threat. I've been running the Calm Mind set, for the reason that it outclasses Calm Mind Cresselia with Thunderbolt instead of HP [Electric] or Charge Beam, and Recover over Moonlight. So far, it's been able to set up in about 50% of battles, and when it has, nothing can reliably wall it barring Toxic or a lucky T-wave Bliss. Deoxys-LG can also run Spikes, Stealth Rock, or Knock Off, moves Cresselia wants but cannot have (well, even if it could have it, it's stuck with Resttalking because of how bad Moonlight is.) There's also the Taunt/Agility/Recover/Toxic set that's run in ubers, although I'm not too sure how effective it would be in SP with all of the Steel-types running around. In fact, Taunt in general is a great move for a wall. I'm also interested in seeing if people are going to start packing Counter or Mirror Coat to beat Tyranitar or Gengar respectively.
The Latis are not as big of a threat as you would expect without Soul Dew. Every main set barring the DD Latios set is walled soundly by any Blissey with a status move; of course, I'm just waiting for Refresh or Safeguard Lati@s to pop up, then I'm expecting Steel Types to give it trouble. The DD set is pathetically walled by normal DD set counters (I'm looking at you, Skarm and Zong,) because let's face it, Latios' 306 Atk is nothing special (That's with a positive nature...) Latios' most damaging set is the Calm Mind+Draco Meteor set with Life Orb, but again, this is walled easily by Blissey and a few other special walls. Latias is effectively an inferior Latios, although I'm interested in seeing if someone is going to combine Latios+Blissey to make a pseudo-wall, pseudo-sweeper with Wish support, as I feel this set would be most effective for Latias.
Tyranitar is a great check for all four of these debatable Pokemon, and of course I'm talking the same set that it uses in ubers to beat them; 252 HP, 216 SpDef, 40 Atk, Careful, Stone Edge/Earthquake/Crunch/Pursuit. Crunch will OHKO 4 HP/0 Def Latios or Latias, as will Pursuit if either of them decide to switch out. Tyranitar can switch into anything from them with the exception of Choice Specs HP [Fighting] or Grass Knot and DD Earthquake. Crunch also soundly 2HKOes Deoxys-LG, although it will get 3 attacks on Tyranitar if it has to switch in, meaning LG can set up SR on the switch, paralyze T-tar after the first attack, and either attempt to Recover stall or switch out, leaving the opponent with a crippled counter. Tyranitar's CB Stone Edge does around 75% to 4 HP/0 Def Manaphy, meaning that T-tar cannot counter Manaphy nor revenge kill Manaphy at full health. Manaphy will 2HKO with a Life Orb Tail Glow Surf, or OHKO if the attack is Hydro Pump.
Some other general counters for the Lati@s are Shuckle, who can take any hit easily in Sandstorm and Encore the twins, Knock Off their item, or Toxic them, Bronzong, who can put them to sleep or take off a chunk of health with Gyro Ball, Metagross, who can take any hit aside from DD EQ and OHKO with CB Meteor Mash or simply Pursuit them, the comedy Shedinja mention, as Hidden Power [Fire] isn't too common, Regice, Jirachi.
Wobbuffet can switch into any Deoxys-LG set without Toxic and set up on it (is Wobbuffet banned in this tier?) Gengar can switch into a non-status, non-Psychic attack and scare it away immediately. Umbreon can switch in without fear, thanks to Synchronize, and Trap pass to something else if Deoxys is foolish enough to stay in. Blissey is a safe switch-in to Deoxys-LG as well thanks to Natural Cure and legendary Special Defenses, but Deoxys-LG might just use Blissey as a free set-up if she doesn't have Toxic.
Reliable counters for Manaphy are Shedinja and Roar Vaporeon. Less reliable counters: Blissey, who will beat any set without Rest, Ludicolo, who must pack Energy Ball because Manaphy is so light, and must also be wary of CM Manaphy, and...well...this list is pretty short at the moment.
To summarize: From the SP testing done yesterday, I view Manaphy as a huge threat who is likely to be Untiered, Deoxys-LG as a large-but-not-quite-gamebreaking threat, and Lati@s as a great special sweeper and little more.