Below are the moves that ignores stat modifications and damage would remain same.
1. All OHKO moves (They will KO the opponent if it hits^^)
- Fissure
- Guillotine
- Horn Drill
- Sheer Cold
2. Fixed damaging moves^ (not OHKO)
- Dragon Rage
- Sonic Boom
- Endeavour
- Super Fang
- Seismic Toss
- Night shade
- Nature's madness
- Final Gambit
- Curse (When used by Ghost type, opponent loses 25% of Maximum HP each turn)
- Guardian of Alola (Cuts the opponent's HP by 75% of it's remaining)
- Leech seed
- Nightmare (Sleeping opponent loses ¼ of it's max. HP each turn)
- Pain split
- Perish song (3rd turn)
- Sand Tomb (After 1st turn of it's most recent usage)
- Sappy seed (After 1st turn of it's most recent usage)
- Magma storm (After 1st turn of it's most recent usage)
3. Moves related to foe's previous attack
- Metal Burst
- Mirror coat
- Psywave
- Counter
- Bide
4. Always critical hitting moves^^^
- Storm Throw
- Frost Breath
- Zippy Zap
- Surging Strikes
- Wicked Blow
Others: Sacred Sword, Darkest lariat, Chip away
^^ Such kind of moves have infinite power. However stat boosts doesn't affect it. These moves fail if it has 'sturdy' ability and items like focus sash.
^ These moves will always cut the opponent's HP in a fixed manner. Stat modifications won't affect their damaging.
^^^ All critical hits and all Always critical-hitting moves Ignores user's negative stat changes and target's positive stat changes.
Thank you!
Source #1 : My experience and this site (Mostly)
Source #2
Source #3
Source - Critical hit
