First this was gonna be a set dump.
Then I was like "lets make a flow chart of when to use a certain mega stone"
Then I was like I'm lazy
So here it is, a guide on how to use any stone, let me know if there are any errors/stuff to add, and if anyone wants to make this into a more readable flow chart, be my guest.
Banned stones
Beedrillite (banned)
Blazikenite (banned)

Gengarite (banned)

Kangaskhanite (banned)
Mawilite (banned)
Medichamite (banned)
Allowed Stones
Abomasite (+40 Attack, +30 Defense, +40 Special Attack, +20 Special Defense, -30 Speed | Snow Warning | N/A | +49.5 Kg)
Want snow? Want no other use?

Absolite (+20 Attack, +40 Special Attack, +40 Speed | Magic Bounce | N/A | +2 Kg)
Special Attackers that can be used a suicide lead with Magic Bounce.

Aerodactylite (+30 Attack, +20 Defense, +10 Special Attack, +20 Special Defense, +20 Speed | Tough Claws | N/A | +20 Kg)
Physical Attackers with lots of contact moves.
Aggronite (+30 Attack, +50 Defense, +20 Special Defense | Filter | - Secondary | +35 Kg)
Useful for defense pokemon to get rid of a type to decrease weaknesses.

Alakazite (+20 Defense,+40 Special Attack, +10 Special Defense, +30 Speed | Trace | N/A | N/A)
Special Attackers which don't have a preferred ability.
Altarianite (+40 Attack, +20 Defense, +40 Special Attack | Pixilate | +
| N/A)
Pokemon with strong normal moves that can abuse Pixilate and fairy type.
Ampharosite (+20 Attack, +20 Defense, +50 Special Attack, +20 Special Defense, -10 Speed | Mold Breaker | +
| N/A)
Bulky Special Attackers that don't need speed. They can use dragon type to their liking. It can also be used as a counter to magic bounce.
Audinite (+40 Defense, +20 Special Attack, +40 Special Defense | Healer | +
| +1 Kg)
Why?
Banettite (+50 Attack, +10 Defense, +10 Special Attack, +20 Special Defense, +10 Speed | Prankster | N/A | +0.5 Kg )
Prankster support pokemon. Also could be used as a suicide lead due to the high attack.

Blastoisinite (+20 Attack, +20 Defense, +50 Special Attack, +10 Special Defense | Mega Launcher | N/A | +15.6 Kg)
Special Attackers with lots of pulse moves.
Cameruptite (+20 Attack, +30 Defense, +40 Special Attack, +30 Special Defense, -20 Speed | Sheer Force | N/A | +100.5 Kg)
Bulky Special Attackers that don't need speed with lots of moves with secondary effects.

Charizardite X (+46 Attack, +33 Defense, +21 Special Attack | Tough Claws | +
| +20 Kg)
Physical Attackers with lots of contact moves.

Charizardite Y (+20 Attack, +50 Special Attack, +30 Special Defense | Drought | N/A | +10 Kg)
Special Fire type Attackers that can abuse their sun while they wait for

Groudon.
Diancite (+60 Attack, -40 Defense, +60 Special Attack, -40 Special Defense, +60 Speed | Magic Bounce | N/A | +19 Kg)
Pokemon that don't give a damn about living a hit. Good suicide lead due to Magic Bounce.
Galladite (+40 Attack, +30 Defense, +30 Speed | Inner Focus | N/A | +4.4Kg)
General use for Physical Attackers.

Garchompite (+40 Attack, +20 Defense, +40 Special Attack, +10 Special Defense, -10 Speed | Sand Force | N/A | N/A)
Rock, Ground, and Steel type attackers that can abuse sand. (if you can get it up)
Gardevoirite (+20 Attack, +40 Special Attack, +20 Special Defense, +20 Speed | Pixilate | N/A | N/A)
Special Attackers with strong normal type moves to abuse Pixilate. Fairy types can make this better.

Glalitite (+40 Attack, +40 Special Attack, +20 Speed | Refrigerate | N/A | +93.7 Kg)
Pokemon with strong normal moves to use. Ice types can make this better.

Gyaradosite (+30 Attack, +30 Defense, +10 Special Attack, +30 Special Defense | Mold Breaker | +
| +70 Kg)
Physical Attackers that can counter Prankster with their dark type and Magic Bounce with Mold Breaker. They are a good counter lead.
Heracronite (+60 Attack, +40 Defense, +10 Special Defense, -10 Speed | Skill Link | N/A | +8.5 Kg)
Physical Attackers with multi-hit moves.
Houndoominite (+40 Defense, +30 Special Attack, +10 Special Defense, +20 Speed | Solar Power | N/A | +14.5 Kg)
Special Fire type Attackers that can abuse sun. They are also waiting for

Groudon.

Latiasite (+20 Attack, +30 Defense, +30 Special Attack, +20 Special Defense | Levitate | N/A | +12 Kg)
Defensive pokemon that are weak to ground.

Latiosite (+40 Attack, +20 Defense, +30 Special Attack, +10 Special Defense | Levitate | N/A | +10 Kg)
Defensive pokemon that are weak to ground which can hit back with physical moves. It can also be used with Physical Attackers that are weak to ground.
Lopunnite (+60 Attack, +10 Defense, +30 Speed | Scrappy | +
| -5 Kg)
Physical Attackers that can use the new fighting type effectively.

Lucarionite (+35 Attack, +18 Defense, +25 Special Attack, +22 Speed | Adaptability | N/A | +3.5 Kg)
Physical Attackers that don't have much coverage or a move that is priority that is the same type as them that they can use.
Manectite (+20 Defense, +30 Special Attack, +20 Special Defense, +30 Speed | Intimidate | N/A | +3.8 Kg)
Defensive pokemon that can act as physical walls with Intimidate and could hit back with special moves.

Metagrossite (+10 Attack, +20 Defense, +10 Special Attack, +20 Special Defense, +40 Speed | Tough Claws | N/A | +392.9 Kg)
Speedy attackers with lots of contact moves. (The only special contact moves are Draining Kiss, Electro Drift, Grass Knot, Infestation, and Petal Dance)
Mewtwonite X (+80 Attack, +10 Defense, +10 Special Defense | Steadfast | +
| +5 Kg)
Use

Lopunnite instead.
Mewtwonite Y (+40 Attack, -20 Defense, +40 Special Attack, +30 Special Defense, +10 Speed | Insomnia | N/A | -89 Kg)
Ummmm...
Pidgeotite (+5 Defense, +65 Special Attack, +10 Special Defense, +20 Speed | No Guard | N/A | +11 Kg)
Special Attackers that have strong moves that aren't 100 accuracy.
why is this not banned yet

Pinsirite (+30 Attack, +20 Defense, +10 Special Attack, +20 Special Defense, +20 Speed | Aerilate | +
| +4 Kg)
Physical Attackers with strong normal type moves to use Aerilate with.

Sablenite (+10 Attack, +50 Defense, +20 Special Attack, +50 Special Defense, -30 Speed | Magic Bounce | N/A | +150 Kg)
Defensive and support leads with Magic Bounce.

Salamencite (+10 Attack, +50 Defense, +10 Special Attack, +10 Special Defense, +20 Speed | Aerilate | N/A | +10 Kg)
If you want the defense I guess?... just use

Pinsirite
Sceptilite (+25 Attack, +10 Defense, +40 Special Attack, +25 Speed | Lightning Rod | +
| +3 Kg)
Special Attackers weak to electric and can use the new dragon type well.

Scizorite (+20 Attack, +40 Defense, +10 Special Attack, +20 Special Defense, +10 Speed | Technician | N/A | +7 Kg)
Pokemon that can abuse low BP moves with Technician.

Sharpedonite (+20 Attack, +30 Defense, +15 Special Attack, +25 Special Defense, +10 Speed | Strong Jaw | N/A | +41.5 Kg)
Pokemon that have lots of biting moves.

Slowbronite (+70 Defense, +30 Special Attack | Shell Armor | N/A | +31.5 Kg)
haha defense go brr. also if you hate crits for some reason.
Steelixite (+40 Attack, +30 Defense, +30 Special Defense | Sand Force | N/A | +340 Kg)
Physical Attackers with lots of Rock, Ground, and Steel moves they can abuse Sand Force with.
Swampertite (+40 Attack, +20 Defense, +10 Special Attack, +20 Special Defense, +10 Speed | Swift Swim | N/A | +20.1 Kg)
Physical Attackers that can abuse Rain. They are waiting for

Kyogre, and then it's inevitable ban.
Tyranitarite (+30 Attack, +40 Defense, +20 Special Defense, +10 Speed | Sand Stream | N/A | +53 Kg)
Set Sand.

Venusaurite (+18 Attack, +40 Defense, +22 Special Attack, +20 Special Defense | Thick Fat | N/A | +55.5 Kg)
Defense pokemon weak to Ice or Fire that can benefit from Thick Fat.