Most powerful entities in the Square Enix Multiverse

[Sephiroth]

Feel free to discuss what you think or just write your list down if you want.

"Warriors of Light": Teamwork of Friendship, Love and everything Good in every Final Fantasy when they all fight together
This is a plot device power that defeats everything. I do not mean "the warriors of light" but the bond of friendship in stories.
Chaos - Final Fantasy Dissidia, probably Final Fantasy, then Garland, too
He possesses the power of Chaos making him equal to Etro.
Cosmos - Final Fantasy Dissidia
She possesses the cosmic power, pretty much the "good" power in general.
Shinryu - Final Fantasy Dissidia
He is on a similiar scale as Chaos as Cosmos with his divine powers.
Mateus Palamecia - Final Fantasy II
He has a strong soul which allows him to continue in the afterlife and bring chaos to it.
Noah/Xande/Doga/Unei - Final Fantasy III
Dark Cloud - Final Fantasy III
Cecil - Final Fantasy IV Saga
Theodore - Final Fantasy IV Saga
FuSoYa - Final Fantasy IV Saga
Zemus - Final Fantasy IV Saga
Ghuido - Final Fantasy V
X-Death - Final Fantasy V
Enuo - Final Fantasy V
The Void - Final Fantasy V
All three entities/natural forces wield or are the void. X-Death and Enuo are on par to each other but they have a weak will and cannot fully control the void - other than that they are next to being the most powerful villains or forces in Final Fantasy.
Terra - Final Fantasy VI
Statues / Kefka - Final Fantasy VI
Sephiroth - Compilation of Final Fantasy VII
He has the most powerful, an indestructable and unbeatable will and while having a variety of abilites that make him incomparably strong anyway and are not allowed to be used all at once because of "plot chance" he possesses the absolute source of power: The stronger the will gets the stronger he gets in general. No other Final Fantasy character was ever defined to be that powerful with his will and with the power concept of Final Fantasy VII he possesses the greatest powers of all. He does not control chaos the void, space or time but he does not need to. His will power is all he needs and it continuesly makes him stronger and his will is strengthened the more energy and knowledge he gets giving him an infinitely running energy source. He can control what he wants with his will and even strengthened his soul more in a moment of weakness before Advent Children making it impossible to fade away. Sephiroth is a plot device-only beatable character and even that is always shown to defeat him because of his own arrogance - which is also part of the plot device as he does not get his job done even though he could easily.
Genesis - Compilation of Final Fantasy VII
Cloud - Compilation of Final Fantasy VII
He possesses the power that makes him on par with SOLDIERs and the canonical "LIMIT Break" power that mobilizes some incomparable "Son-Goku Dragon Fist power" than the other heroes ever had can overwhelm Sephiroth with a surprise attack.
Zack - Compilation of Final Fantasy VII
Vincent - Compilation of Final Fantasy VII
Aerith/Lifestream/Minerva/Omega - Compilation of Final Fantasy VII
Subject JENOVA - Compilation of Final Fantasy VII
The other WEAPONs - Compilation of Final Fantasy VII
Adel - Final Fantasy VIII
Ultimecia - Final Fantasy VIII
Hyne - Final Fantasy VIII
Kuja - Final Fantasy IX
Zidane - Final Fantasy IX
Mikoto - Final Fantasy IX
Necron - Final Fantasy IX
[Sin]/Yevon=Ju/High Summon - Final Fantasy X Saga
Seymour - Final Fantasy X Saga
Yunalesca - Final Fantasy X Saga
Trema - Final Fantasy X Saga
Vayne/Occuria - Ivalice Alliance
Queen Remedi - Ivalice Alliance
Lloyd - Ivalice Alliance
Lightning - Fabula Nova Crystallis
Caius - Fabula Nova Crystallis
Etro - Fabula Nova Crystallis
Muin - Fabula Nova Crystallis
Pulse - Fabula Nova Crystallis
Lindzei - Fabula Nova Crystallis
Bhunivelze - Fabula Nova Crystallis
Orphan/Barthandelus - Fabula Nova Crystallis
Pretty much the gods that represent nature in itself with a few cerain powers. Etro, Lightning, Bhunivelze, Muin, Caius, et cetera. They all are part of the most powerful characters with Lightning being stronger than Cloud normally and as strong as him with his full power.
Kingdom Hearts/Hearts/Light & Darkness - Kingdom Hearts Saga
Master Xehanort/Xemnas/Ansem - Kingdom Hearts Saga
The most powerful entities in Kingdom Hearts. Light and Dark itself and Xehanort, strong enough to wield such powers in their purest form. He has a long story but I don't want to write it down now. However he is more powerful than most Final Fantasy characters.
Sora/Roxas/Xion - Kingdom Hearts Saga
Riku - Kingdom Hearts Saga
Kairi/Naminé - Kingdom Hearts Saga
Terra - Kingdom Hearts Saga
Aqua - Kingdom Hearts Saga
Ventus - Kingdom Hearts Saga
Organisation XIII - Kingdom Hearts Saga
Master Eraqus - Kingdom Hearts Saga
Mickey - Kingdom Hearts Saga
Lavos/Queen Zeal - Chrono Saga
Magus - Chrono Saga
Nier Replicant & Nier Gestalt - Nier
Emil & Halua - Nier
Grimore Weiss & Grimoire Noire - Nier
Missing Procedure/Satanail - Star Ocean
Ethereal Queen & Gabriel Celeste - Star Ocean
Asmodeus - Star Ocean
Arumat - Star Ocean

I don't know all products, those are the ones I have had in mind.