Hello guys, in like 1 month I'll have a tournament, with VGC10 rules with some changes (bold). Up to 6 pokemon can be registered and I did copy/paste from here and edited. I didn't tested it since now that it will have both battle types, is kinda weird to ask everywhere in PO:
Now that you know that twists on the rules (I know, that's weird), and after thinking on the core of the team, asking and reading a lot, I picked Cresselia as the core of my team due for her great support for either singles or double, screen abilities and her teammates will always thank me if I use Stealth Rock as a entry hazard, so, my lead is:
Forretress @Quick Claw (Only for single battle rounds)
Nature: Relaxed
Ability: Sturdy
EVs 252 HP / 4 Def / 252 SpD
~ Stealth Rock
~ Toxic Spikes
~ Gyro Ball
~ Explosion
In a past tournament, most people didn't have much knowledge of leads, so I can and the point is really just to set up hazards, and check if my opponents Pokémon are weak to Gyro Ball massive damage thanks to its crappy speed. Explosion for desperate situations or having low hp.
Cresselia @Light Clay
Nature: Bold
Ability: Levitate
EVs 252 HP / 4 Def / 252 SpD
~ Light Screen
~ Hidden Power Fighting
~ Thunder Wave
~ Lunar Dance
Here comes the star! I picked up Cresselia for its great defensive stats which can do a hard time to my opponent thanks to her support and defense, either in doubles or singles. The idea is use LS and switch out, to save her for Lunar dance later to heal one of her partners completely. HP Fighting is for Dark type Pokémon that comes against her, and a great partner for her is this guy:
Groudon @Life Orb
Nature: Jolly
Ability: Drought
EVs 112 HP / 252 Atk / 8 SpD / 136 Spe
~ Rock Polish
~ Earthquake
~ Rock Slide
~ Dragon Claw
Any sweeper will thank Forretress' hazards and Groudon is not the exception. With Rock Polish I can outrun things like a Jolly Scarfed Garchomp or an Adamant Rayquaza with no DD. Earthquake to handle Steel pokemon like Dialga and Metagross and Rock Slide for double battles and also deal good damage to flying Pokemon like Rayquaza, Togekiss, Zapdos, Salamence and Gyarados. Threats like Suicune, Kyogre, Lugia and Vaporeon can be damaged by Dragon Claw, or by:
Kyogre @Lum Berry
Nature: Modest
Ability: Drizzle
EVs: 104 HP / 154 SpA / 252 Spe
~ Calm Mind
~ Thunder
~ Surf
~ Ice Beam
This is like a triangle love of Kyogre, Groudon and Cresselia xD I opted for the CM Version to do a lot of damage, also to handle Sandstorm teams that comes to ruin Drought. A powerful STAB after a CM, Thunder to things like Skarmory and Palkia. Those speed IVs to outrun Rock Polish Groudon, Palkia and Dialga.
Ludicolo @Leftovers
Nature: Calm
Ability: Rain Dish
EVs 252 HP / 56 Def / 200 SpD
~ Grass Knot
~ Ice Beam
~ Leech Seed
~ Protect
Useful in doubles and singles, rain or sun, this Ludicolo can have a fun time for unprepared teams. Ice beam for dealing with Pokémon like Togekiss, Zapdos, and dragons. Grass Knot for STAB, can do huge damage to Dialga, Palkia, Vaporeon, Suicune, OHKO Swampert. Defensive build to stall using Leech Seed + Protect, which is also useful when I pair it with Groudon's Earthquake on double battle.
Scizor @Choice Band (only for single battle)
Nature: Adamant
Ability: Technician
EVs: 200 HP / 56 Atk / 252 SpD
~ U-turn
~ Bullet Punch
~ Superpower
~ Pursuit
The Choice Band variant never let me down, the EV variant is made to be able to fight against an Uber pokemon. U-Turn for handling ones like Wobbuffet, Lugia, Starmie and Mewtwo. Revenge killing and late game sweeping with Bullet Punch, Superpower to handle stalls like Blissey, Snorlax, Dialga. Pursuit for scary pokemon, also it can do great damage on Giratina, Rotom and Gengar.
I avoided the use of a dragon since I expect most of them: Garchomp, Giratina, Salamence, Dragonite, Rayquaza, Dialga, Palkia... Thanks in advice for helping me out with your suggestions and sorry for my bad english. Did I mentioned that one prize for the winner is a retail game of Black or White? That's why I made this RMT :)
1. HEART GOLD/SOUL SILVER ARE THE ONLY GAMES BEING USED
2. 4 Rounds in swiss format: 2 rounds of Single Battle AND 2 rounds of Double Battle.
3. 6 Pokemon in Single Battle (or 4, not confirmed yet) and 4 Pokemon in Double battle.
4. Pokemon lv. 1 through lv 100 are allowed (!)
5. Battles are at lv. 1-50. If the pokemon are above lv. 50, the game will auto level them down to lv. 50 for the battle. Moves will stay intact during the auto leveling. If a pokemon is under lv. 50, no change is made.
6. No two of the same pokemon can be used (species clause)
7. Item Clause is on, Soul Dew is banned.
8. BANNED POKEMON ARE AS FOLLOWS:
Mew
Celebi
Jirachi
Deoxys (all forms)
Phione
Manaphy
Darkrai
Shaymin
Arceus
9. You may use UP TO TWO UBERS PER BATTLE This means that you cannot have a team like Kyogre/Palkia/Dialga/Mewtwo in one battle. Non-banned Ubers are considered to be:
Mewtwo
Lugia
Ho-oh
Kyogre
Groudon
Rayquaza
Dialga
Palkia
Giratina
10. Any other pokemon may be used (this includes Tyranitar and all Rotom forms!)
11. Maximum 3 Legendaries can be registered (so, that means 2 Ubers and 1 legend that's not on the list, Cresselia or Heatran by example)
2. 4 Rounds in swiss format: 2 rounds of Single Battle AND 2 rounds of Double Battle.
3. 6 Pokemon in Single Battle (or 4, not confirmed yet) and 4 Pokemon in Double battle.
4. Pokemon lv. 1 through lv 100 are allowed (!)
5. Battles are at lv. 1-50. If the pokemon are above lv. 50, the game will auto level them down to lv. 50 for the battle. Moves will stay intact during the auto leveling. If a pokemon is under lv. 50, no change is made.
6. No two of the same pokemon can be used (species clause)
7. Item Clause is on, Soul Dew is banned.
8. BANNED POKEMON ARE AS FOLLOWS:
Mew
Celebi
Jirachi
Deoxys (all forms)
Phione
Manaphy
Darkrai
Shaymin
Arceus
9. You may use UP TO TWO UBERS PER BATTLE This means that you cannot have a team like Kyogre/Palkia/Dialga/Mewtwo in one battle. Non-banned Ubers are considered to be:
Mewtwo
Lugia
Ho-oh
Kyogre
Groudon
Rayquaza
Dialga
Palkia
Giratina
10. Any other pokemon may be used (this includes Tyranitar and all Rotom forms!)
11. Maximum 3 Legendaries can be registered (so, that means 2 Ubers and 1 legend that's not on the list, Cresselia or Heatran by example)
Now that you know that twists on the rules (I know, that's weird), and after thinking on the core of the team, asking and reading a lot, I picked Cresselia as the core of my team due for her great support for either singles or double, screen abilities and her teammates will always thank me if I use Stealth Rock as a entry hazard, so, my lead is:

Forretress @Quick Claw (Only for single battle rounds)
Nature: Relaxed
Ability: Sturdy
EVs 252 HP / 4 Def / 252 SpD
~ Stealth Rock
~ Toxic Spikes
~ Gyro Ball
~ Explosion
In a past tournament, most people didn't have much knowledge of leads, so I can and the point is really just to set up hazards, and check if my opponents Pokémon are weak to Gyro Ball massive damage thanks to its crappy speed. Explosion for desperate situations or having low hp.

Cresselia @Light Clay
Nature: Bold
Ability: Levitate
EVs 252 HP / 4 Def / 252 SpD
~ Light Screen
~ Hidden Power Fighting
~ Thunder Wave
~ Lunar Dance
Here comes the star! I picked up Cresselia for its great defensive stats which can do a hard time to my opponent thanks to her support and defense, either in doubles or singles. The idea is use LS and switch out, to save her for Lunar dance later to heal one of her partners completely. HP Fighting is for Dark type Pokémon that comes against her, and a great partner for her is this guy:

Groudon @Life Orb
Nature: Jolly
Ability: Drought
EVs 112 HP / 252 Atk / 8 SpD / 136 Spe
~ Rock Polish
~ Earthquake
~ Rock Slide
~ Dragon Claw
Any sweeper will thank Forretress' hazards and Groudon is not the exception. With Rock Polish I can outrun things like a Jolly Scarfed Garchomp or an Adamant Rayquaza with no DD. Earthquake to handle Steel pokemon like Dialga and Metagross and Rock Slide for double battles and also deal good damage to flying Pokemon like Rayquaza, Togekiss, Zapdos, Salamence and Gyarados. Threats like Suicune, Kyogre, Lugia and Vaporeon can be damaged by Dragon Claw, or by:

Kyogre @Lum Berry
Nature: Modest
Ability: Drizzle
EVs: 104 HP / 154 SpA / 252 Spe
~ Calm Mind
~ Thunder
~ Surf
~ Ice Beam
This is like a triangle love of Kyogre, Groudon and Cresselia xD I opted for the CM Version to do a lot of damage, also to handle Sandstorm teams that comes to ruin Drought. A powerful STAB after a CM, Thunder to things like Skarmory and Palkia. Those speed IVs to outrun Rock Polish Groudon, Palkia and Dialga.

Ludicolo @Leftovers
Nature: Calm
Ability: Rain Dish
EVs 252 HP / 56 Def / 200 SpD
~ Grass Knot
~ Ice Beam
~ Leech Seed
~ Protect
Useful in doubles and singles, rain or sun, this Ludicolo can have a fun time for unprepared teams. Ice beam for dealing with Pokémon like Togekiss, Zapdos, and dragons. Grass Knot for STAB, can do huge damage to Dialga, Palkia, Vaporeon, Suicune, OHKO Swampert. Defensive build to stall using Leech Seed + Protect, which is also useful when I pair it with Groudon's Earthquake on double battle.

Scizor @Choice Band (only for single battle)
Nature: Adamant
Ability: Technician
EVs: 200 HP / 56 Atk / 252 SpD
~ U-turn
~ Bullet Punch
~ Superpower
~ Pursuit
The Choice Band variant never let me down, the EV variant is made to be able to fight against an Uber pokemon. U-Turn for handling ones like Wobbuffet, Lugia, Starmie and Mewtwo. Revenge killing and late game sweeping with Bullet Punch, Superpower to handle stalls like Blissey, Snorlax, Dialga. Pursuit for scary pokemon, also it can do great damage on Giratina, Rotom and Gengar.
I avoided the use of a dragon since I expect most of them: Garchomp, Giratina, Salamence, Dragonite, Rayquaza, Dialga, Palkia... Thanks in advice for helping me out with your suggestions and sorry for my bad english. Did I mentioned that one prize for the winner is a retail game of Black or White? That's why I made this RMT :)