Genesect C+ -> B-
I'm honestly surprised this is ranked so low. Genesect fulfills a variety of rolls, it is one of the only Pokemon that can check Darkrai if it has a Substitute up, and more viable than most of the alternatives (looking at you Ninjask and Sylveon). It is also an excellent pivot, with U-turn it can effectively take advantage of checks such as Groudon or Ho-Oh when paired with a strong wallbreaker like Primal Kyogre. Genesect's last moveslot is generally quite open so it can be customized to fit a team's needs, Ice Beam lets it threaten Rayquaza and do respectable damage to Groudon while other options include Extreme Speed, Flamethrower or Explosion.
Anyways, here's another team you might want to consider for the sample teams archive, I'm surprised nobody's submitted something like it yet.
Not a Llama (Rayquaza-Mega) @ Choice Band
Ability: Delta Stream
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Dragon Ascent
- V-create
- Waterfall
- Extreme Speed
Llama (Arceus) @ Lum Berry
Ability: Multitype
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Extreme Speed
- Earthquake
- Magic Coat
- Swords Dance
Llama (Arceus) @ Life Orb
Ability: Multitype
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Extreme Speed
- Earthquake
- Fire Blast
- Swords Dance
Pink Llama (Arceus-Fairy) @ Pixie Plate
Ability: Multitype
EVs: 248 HP / 44 Def / 216 Spe
Timid Nature
IVs: 0 Atk
- Judgment
- Will-O-Wisp
- Recover
- Defog
Australian Llama (Arceus-Poison) @ Toxic Plate
Ability: Multitype
EVs: 248 HP / 120 SpA / 40 SpD / 100 Spe
Timid Nature
IVs: 0 Atk
- Judgment
- Earth Power
- Psych Up
- Recover
Llama (Arceus) @ Rocky Helmet
Ability: Multitype
EVs: 248 HP / 244 Def / 16 Spe
Impish Nature
- Extreme Speed
- Recover
- Stealth Rock
- Roar
Ability: Delta Stream
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Dragon Ascent
- V-create
- Waterfall
- Extreme Speed
Llama (Arceus) @ Lum Berry
Ability: Multitype
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Extreme Speed
- Earthquake
- Magic Coat
- Swords Dance
Llama (Arceus) @ Life Orb
Ability: Multitype
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Extreme Speed
- Earthquake
- Fire Blast
- Swords Dance
Pink Llama (Arceus-Fairy) @ Pixie Plate
Ability: Multitype
EVs: 248 HP / 44 Def / 216 Spe
Timid Nature
IVs: 0 Atk
- Judgment
- Will-O-Wisp
- Recover
- Defog
Australian Llama (Arceus-Poison) @ Toxic Plate
Ability: Multitype
EVs: 248 HP / 120 SpA / 40 SpD / 100 Spe
Timid Nature
IVs: 0 Atk
- Judgment
- Earth Power
- Psych Up
- Recover
Llama (Arceus) @ Rocky Helmet
Ability: Multitype
EVs: 248 HP / 244 Def / 16 Spe
Impish Nature
- Extreme Speed
- Recover
- Stealth Rock
- Roar
Most people have seen a team like this before (in fact I've come across teams almost exactly like this on the ladder), Godspam is a really strong archetype. This is mostly a team I use for blasting through low ladder but it works very well against skilled opponents too, it's very powerful and quite well balanced overall. Using this team isn't too complicated, use Rayquaza to bust holes in the opposing team so an EKiller can sweep, falling back on the three remaining Arceus if the team needs to go on the defensive. The EKiller sets can easily be customized to personal preference, Magic Coat+Lum Berry EKiller is a better Darkrai/Klefki check and also can bounce back status/phazing moves from Lugia or Giratina. Life Orb EKiller with Fire Blast is my preference to lure Skarmory while still being a very threatening sweeper. Fairy Arceus beats Yveltal and Rayquaza, keeps hazards off the field and can spread burns to weaken opponents. Bulky Arceus with Rocky Helmet lets the team have an advantage over opposing EKiller spam teams and provides the ever needed Stealth Rocks. Lastly, Psych Up Poisonceus is a dank meme that checks Xerneas really well and can threaten a countersweep while also being useful against Clefable and opposing Fairy Arceus. There's not too much that can give this team a lot of trouble, Ghostceus can be tricky and Stall Mewtwo gave me heck once but otherwise this team has decent matchups against the whole metagame. Just keep your defensive answers to things alive, otherwise the team is quite vulnerable to setup sweepers.