All right, so here's my headcanon from the current roster.
Though granted, my list takes into factor their general behavior alongside their raw power.
LEGENDS:
Disaster Level: This measures the destructive capacity of a monster or seijin. Kaiju are ranked at levels between 1-5, and rarely going beyond that. This is not to be confused with Classification, which systematically categorizes kaiju based on their behavior.
Classification: This indicates specialized characteristics that place kaiju in categories depending on their intellect and/or behavior.
Asterisk (*) - Is used in conjuncture with specialized transformations or a different creature of the same species, which can have different outcome with Disaster Levels and Classifications.
Classification: Apocalyptic-type - They destroy to destroy, or have the capacity to do so. These monsters have the ability to wipe out the face of a planet if left unabated.
*Bagan [Level 5+]
*Burning Godzilla [Level 5+]
Legion [Level 5+]
*Bagan (Energy Cocoon) [Level 5+]
Grand King Ghidorah [Level 5+]
SpaceGodzilla [Level 5]
Destoroyah [Level 5]
Dogora [Level 5]
*Keizer Ghidorah [Level 5]
Orochi (Heisei) [Level 5]
Desghidorah [Level 4]
Hedorah (Showa) [Level 4]
Classification: Warrior-type - Often possessing some form of intellect, be it truly sentient or primal, Warrior-types are known for their fierce wills, tenacity, and combat adaptability. They either fight with a mission or just fight for the thrill of it.
*Super Godzilla [Level 5+]
Krystalak [Level 5]
*Godzilla (Heisei) [Level 5]
Mechagodzilla (Showa) [Level 4]
Megalon [Level 4]
King Ghidorah (Heisei) [Level 4]
Dagahra [Level 4]
Biollante [Level 4]
Mechani-Kong [Level 4]
*Monster X [Level 4]
Orga [Level 4]
Jikiro [Level 3]
Iris [Level 3]
Battra [Level 3]
Gyaos (Heisei) [Level 3]
*Gigan (Modified) [Level 3]
*Gigan (Millennium) [Level 3]
Kumasogami [Level 3]
Godzilla (Legendary) [Level 3]
Colossal Titan [Level 3]
*Gigan (Showa) [Level 3]
Gororin [Level 3]
Wargilgar [Level 2]
Zandora [Level 2]
Obsidius [Level 2]
Cyber Godzilla [Level 2]
Anguirus (Showa) [Level 2]
Spyler [Level 2]
King Kong [Level 2]
Black Moth [Level 2]
Classification: Beast-type - Wild, primal, and savage; by every definition, these are often seen as the "giant animal" types of the kaiju.
Rodan (Heisei) [Level 4]
Titanosaurus [Level 4]
Megaguirus [Level 4]
Goliath [Level 3]
Gomora [Level 3]
*M.U.T.O. (Female) [Level 3]
Red King [Level 3]
Bemular [Level 2]
Kamacuras (Millennium) [Level 2]
*M.U.T.O. (Male) [Level 2]
Skullcrawler [Level 2]
C-Rex [Level 1]
Baragon (Showa) [Level 1]
Gabara [Level 1]
Gorosaurus [Level 1]
Varan [Level 1]
Maguma [Level 1]
Manda (Millennium) [Level 1]
*Zilla [Level 1]
Kumonga (Showa) [Level 1]
Ebirah (Millennium) [Level 1]
Gezora [Level 1]
Gaira (Showa) [Level 1]
Godzillasaurus [Level 1]
Classification: Exempt-types - These types are often applied to Mecha units, though they can also apply to those who have proven not to be a threat to humanity (besides collateral damage).
*Armor Mothra [Level 5+]
Utsuno Ikusagami [Level 5+]
*Rainbow Mothra [Level 5]
Moguera (Heisei) [Level 5]
Mecha-King Ghidorah [Level 5]
*Aqua Mothra [Level 5]
Mothra Leo [Level 4]
Gamera (Heisei) [Level 4]
Zone Fighter [Level 4]
Kiryu [Level 4]
Mechagodzilla (Heisei) [Level 4]
Ultraman [Level 4]
Ultraseven [Level 4]
Mothra (Heisei) [Level 3]
Godzilla Junior [Level 3]
Jet Jaguar [Level 3]
Rogue Titan [Level 2]
*Zilla Junior [Level 2]
King Caesar (Millennium) [Level 2]
Daigoro [Level 1]
Sanda [Level 1]
Frankenstein [Level 1]
*NOTE: Still gotta update more with rankings to other transformations/"forms" and chronological order to how strong I see each of the monsters, even if the margin isn't by a lot. If you feel inclined to disagree with how I categorized or ranked some of the monsters, feel free to let me know!
I know, right? I can't believe it either! 
