I like this team a lot and it has gone through extensive tweaking, but it has some pretty gaping weaknesses. Still, it's very competitive for something without the usual Giratina, Kyogre, etc.
Latios @ Soul Dew
Timid, 6 HP/252 Speed/252 Sp. Attack
Draco Meteor
Thunder
Grass Knot
Memento
I started off the team based around wanting to use Latios as a starter, and this guy is a real champion. Memento is to give a near dead Latios something to do after a Draco Meteor kill. It may seem like a useless move, but it sets up Tyranitar to survive a hit while Rock Polishing.
Mewtwo @ Choice Specs
Timid, 42 HP/216 Speed/252 Sp. Attack
Aura Sphere
Ice Beam
Shadow Ball
Thunder
Basically the uber equivalent of Specs Lucario, except with speed to back it up. He is an excellent anchor to the team and can force switches, revenge kill, and even sweep in the later stages of the match. Also, he has an easy time finding opportunities to come in off a slow Scizor U-Turn or after Smeargle dies.
Rayquaza @ Choice Band
Adamant, 252 Attack/252 Speed/6 Sp. Def
Outrage
Extremespeed
Earthquake
Rain Dance
Choice Banded Outrage has a really good chance of killing Giratina, who causes major problems for this team. Extremespeed let's him pseudo-revenge kill as well. Rain Dance is a filler move to clear an opposing Sandstorm to allow Smeargle to operate.
Scizor (Technician) @ Expert Belt
Adamant, 176 HP/100 Attack/56 Speed/176 Sp. Defense
Pursuit
Quick Attack
Roost
U-Turn
This guy fills a much needed void in the team, being able to come in on quite a few attacks and hit hard from the physical side. I don't really know what else to say, it's pretty straight-forward.
Smeargle (level 5) @ Focus Sash
Natures/IV don't matter
Trick Room
Spore
Endeavor
Dragon Rage
This guy really wreaks havoc on unprepared teams. Stealth Rock completely destroys him, but once you clear their team of Giratina (no easy feat) then little stands in Smeargle's way. Everyone has seen it before, but of course you bring him in midgame after someone dies, live through the hit and Trick Room, Spore, and proceed to Endeavor/Dragon Rage. Usually Endeavoring twice in a row is the best route, because even if they don't switch out you'll be able to finish them off with Sandstorm.
Tyranitar @ Life Orb
Hasty, 232 Attack/48 Speed/228 Sp. Attack
Rock Polish
Crunch
Ice Beam
Flamethrower
This guy really ties everything together. Ideally, he comes out as a surprise in the very late game, after Smeargle has done his thing. First, Sandstorm will finish off anyone who was hit with Endeavor. Next, he should be able to get a Rock Polish off if he was set up by a Latios Memento. The nature/EV spread allows you to get to 374 after a speed boost, outspeeding stuff like Darkrai. Even without Rock Polish sweeping, he's still very sturdy and can switch in on a good number of attacks. I used Pursuit over Flamethrower for a while, but either way you'll mainly be Crunching/Ice Beaming. If anyone has a superior nature/EV spread suggestion for him, I'd love to hear it as I'm not entirely convinced the one I have is optimal.
On a side note, this team unintentionally meets the Item Clause. Darkrai starters (which have seemingly increased in popularity as of late) are really difficult for this team to play against, as are most status effects in general since nobody on the team has Rest or Taunt. Anyone who poses the threat of Stealth Rocking requires extreme prediction to play against, though sometimes you can still salvage Smeargle by switching him in on the turn that they use it. Anything that hits hard and has Choice Scarf also is a pain, since Mewtwo and Latios rely on outspeeding things with max speed natures since their special attacks are already buffed by items. Giratina is the number one threat to this team since he shuts down Smeargle completely, wrecks Tyranitar with a WoW, and can come in on an immune attack from both choice users. Due to this, the three main early game attackers all feature a move to hurt him severely (Latios -Draco Meteor, Rayquaza- Outrage, Mewtwo- Ice Beam), so with a little bit of prediction you can put a serious dent in him.
Overall, it requires a lot of prediction skills (and quite a bit of luck) due the majority of the team being a) relatively fragile or b) choice locked, but it is a very competitive team considering that it has three non-legends.
Latios @ Soul Dew
Timid, 6 HP/252 Speed/252 Sp. Attack
Draco Meteor
Thunder
Grass Knot
Memento
I started off the team based around wanting to use Latios as a starter, and this guy is a real champion. Memento is to give a near dead Latios something to do after a Draco Meteor kill. It may seem like a useless move, but it sets up Tyranitar to survive a hit while Rock Polishing.
Mewtwo @ Choice Specs
Timid, 42 HP/216 Speed/252 Sp. Attack
Aura Sphere
Ice Beam
Shadow Ball
Thunder
Basically the uber equivalent of Specs Lucario, except with speed to back it up. He is an excellent anchor to the team and can force switches, revenge kill, and even sweep in the later stages of the match. Also, he has an easy time finding opportunities to come in off a slow Scizor U-Turn or after Smeargle dies.
Rayquaza @ Choice Band
Adamant, 252 Attack/252 Speed/6 Sp. Def
Outrage
Extremespeed
Earthquake
Rain Dance
Choice Banded Outrage has a really good chance of killing Giratina, who causes major problems for this team. Extremespeed let's him pseudo-revenge kill as well. Rain Dance is a filler move to clear an opposing Sandstorm to allow Smeargle to operate.
Scizor (Technician) @ Expert Belt
Adamant, 176 HP/100 Attack/56 Speed/176 Sp. Defense
Pursuit
Quick Attack
Roost
U-Turn
This guy fills a much needed void in the team, being able to come in on quite a few attacks and hit hard from the physical side. I don't really know what else to say, it's pretty straight-forward.
Smeargle (level 5) @ Focus Sash
Natures/IV don't matter
Trick Room
Spore
Endeavor
Dragon Rage
This guy really wreaks havoc on unprepared teams. Stealth Rock completely destroys him, but once you clear their team of Giratina (no easy feat) then little stands in Smeargle's way. Everyone has seen it before, but of course you bring him in midgame after someone dies, live through the hit and Trick Room, Spore, and proceed to Endeavor/Dragon Rage. Usually Endeavoring twice in a row is the best route, because even if they don't switch out you'll be able to finish them off with Sandstorm.
Tyranitar @ Life Orb
Hasty, 232 Attack/48 Speed/228 Sp. Attack
Rock Polish
Crunch
Ice Beam
Flamethrower
This guy really ties everything together. Ideally, he comes out as a surprise in the very late game, after Smeargle has done his thing. First, Sandstorm will finish off anyone who was hit with Endeavor. Next, he should be able to get a Rock Polish off if he was set up by a Latios Memento. The nature/EV spread allows you to get to 374 after a speed boost, outspeeding stuff like Darkrai. Even without Rock Polish sweeping, he's still very sturdy and can switch in on a good number of attacks. I used Pursuit over Flamethrower for a while, but either way you'll mainly be Crunching/Ice Beaming. If anyone has a superior nature/EV spread suggestion for him, I'd love to hear it as I'm not entirely convinced the one I have is optimal.
On a side note, this team unintentionally meets the Item Clause. Darkrai starters (which have seemingly increased in popularity as of late) are really difficult for this team to play against, as are most status effects in general since nobody on the team has Rest or Taunt. Anyone who poses the threat of Stealth Rocking requires extreme prediction to play against, though sometimes you can still salvage Smeargle by switching him in on the turn that they use it. Anything that hits hard and has Choice Scarf also is a pain, since Mewtwo and Latios rely on outspeeding things with max speed natures since their special attacks are already buffed by items. Giratina is the number one threat to this team since he shuts down Smeargle completely, wrecks Tyranitar with a WoW, and can come in on an immune attack from both choice users. Due to this, the three main early game attackers all feature a move to hurt him severely (Latios -Draco Meteor, Rayquaza- Outrage, Mewtwo- Ice Beam), so with a little bit of prediction you can put a serious dent in him.
Overall, it requires a lot of prediction skills (and quite a bit of luck) due the majority of the team being a) relatively fragile or b) choice locked, but it is a very competitive team considering that it has three non-legends.