In this research i'm using only one weapon as a test and its a Longsword (LS).
The DAMAGE STACKING may be VARIANT from DIFFERENT WEAPON and such,
But this is the ROUGHLY point about damage stacking that i got from my research.

• Powercharm
  
• Powertalon
  
• Powercharm+Powertalon
  
• Canteen meal attack boost (S)
  
• Canteen meal attack boost (M)
  
• Canteen meal attack boost (L)
  
• Demondrug/Nitroshroom
  
• Mega Demondrug
  
• Demonpowder
  
• Mightseed
  
• Mightpill
  
• Weapon charge
  
• Other buff from skills

• Powercharm give +20 base attack.
  
• Powertalon give +30 base attack.

Both items have infinite duration as long you have them in your items pouch.

• CAN BE STACKED

Bringing both items, resulting in +50 base attack.

Having more than 1 of powercharm or powertalon doesn't give you additional attack buff.

You can eat in the canteen to get attack up boost.
This effect last until you DIED or LEAVING THE QUEST(finish, abandoned, return).

• Canteen meal attack up boost (S) give +17 base attack. (need 2 meat).
  
• Canteen meal attack up boost (M) give +33 base attack. (need 4 meat).
  
• Canteen meal attack up boost (L) give +50 base attack. (need 6 meat).

• YOU NEED TO PICK ONE BUFF
  
• CANNOT BE STACKED
  
• CANNOT BE REPLACED

WARNING: if you pick quick meat meal in canteen selection, your health maybe not getting buff into max possible, you you need either Mega Nutrient, Max Potion or Ancient Potion to buff your health to max.

• Demondrug(*Nitroshroom) give +17 base attack.
  
• Mega Demondrug give +23 base attack.

This effect last until you DIED or LEAVING THE QUEST(finish, abandoned, return).

• YOU NEED TO PICK ONE BUFF
  
• CANNOT BE STACKED
  
• Can be replaced if you miss consume Demondrug and want to replace the effect with Mega Demondrug, simply just consume Mega Demondrug.

Tips; Many people prefer using Demondrug since Mega Demondrug is crazy expensive to make, and those resource can be relocated into Mightpills. and some people also using Mushroomancer to be able to eat Nitroshroom.

*Mushroomancer skill level 2

• Demonpowder give +33 base attack.

This effect last for 3 minute.

• Mightseed give +33 base attack. (3mins duration).
  
• Mightpill give +82 base attack, (20s duration).

You could use Mightpill when the monster is downed to deal max damage possible.

• YOU NEED TO PICK ONE BUFF
  
• CANT BE STACKED
  
• Can be replaced if you already consume Mightseed and want to change into Mightpill, simply consume Mightpill.(vice versa)

Tips; some people say the mightpill damage is not worth the crafting since its so expensive, but if you want to deal the max damage possible then nothing is too expensive as long as you know when to use it.

In this topic i only do research using Longsword(LS).
Result may be variant from different weapons.

Charge color:

• LV-0 = TRANSPARENT
  
• LV-1 = WHITE
  
• LV-2 = ORANGE
  
• LV-3 = RED

Charge damage:

• LV-0 give +0 base damage.
  
• LV-1 give +31 base damage.
  
• LV-2 give +79 base damage.
  
• LV-3 give +158 base damage.

This effect last until the charge is worn off.

RAW weapon sharpness multiplier:

• Red = 0,50
  
• Orange = 0,75
  
• Yellow = 1,00
  
• Green = 1,05
  
• Blue = 1,20
  
• White = 1,32
  
• Purple = 1,39

ELEMENTAL weapon sharpness multiplier:

• Red = 0,25
  
• Orange = 0,50
  
• Yellow = 0,75
  
• Green = 1,00
  
• Blue = 1,0625
  
• White = 1,125
  
• Purple = 1,25

Disclaimer; i'm not researching about this one. this sharpness article is from Fextralife;
https://monsterhunterworld.wiki.fextralife.com/Sharpness

• Hammer (x 5.2)
  
• Great Sword (x 4.8)
  
• Hunting Horn (x 4.2)
  
• Charge Blade (x 3.6)
  
• Switch Axe (x 3.5)
  
• Long Sword (x 3.3)
  
• Insect Glaive (x 3.1)
  
• Lance (x 2.3)
  
• Gunlance (x 2.3)
  
• Heavy Bowgun (x 1.5)
  
• Sword and Shield (x 1.4)
  
• Dual Blades (x 1.4)
  
• Light Bowgun (x 1.3)
  
• Bow (x 1.2)

Disclaimer; im not researching about this one. i get it from Fextralife.
https://monsterhunterworld.wiki.fextralife.com/Attack+Power

I'm using Longsword Aspara Glacia, which a random weapon that i pick for the test and not the strongest weapon in the game.

• Bring powercharm and powertalon in pouch.
  
• Consume meat meal attack boost (L).
  
• If health not max, consume Max Potion.
  
• Consume Demondrug or Mega Demondrug.
  
• When closing the monster use Demonpowder and Mightseed.
  
• If monster downed use Mightpills
  
• Reapply Mightseed after Mightpills effect has worn off.
  
• Keep hitting the monster.
  
• Dont die.
  
• Git gud.

Any skills that improve attack/base damage can be use to deal even more damage.
e.g;

• Attack boost,
  
• Agitator,
  
• Heroics,
  
• Peak Perfomance,
  
• Resentment,
  
• Partbreaker,
  
• Critical boost, etc