For my own interest, I'm trying to create a list of the strongest (read: highest CR) creature of every creature type. Based on some responses in the comments (see end of post), I've divided it into categories. Neither list will feature 3pp or creatures with class levels (or templates), since that makes the whole thing trivial. I'm also not including things like Empyrean Paragons and Elder Jinushigami, which have CR's but no full statblocks (though I am including great wyrm dragons, since there are comprehensive rules for what their statblocks would be).

So far, from scouring Nethys, I've got:

UNIQUE

• Aberration: Great Old One Cthulhu^M, Great Old One Mordiggian^M (CR 30)

• Dragon: Fafnheir, Shen^M (CR 24)

• Magical Beast: Leviathan, Varklops (kaiju) (CR 30)

• Monstrous Humanoid: Great Old One Ithaqua^M (CR 28)

• Ooze: Vorgozen (kaiju) (CR 29)

• Outsider: Achaekek the Mantis God^3.5, Archdevil Baalzebul^M, Archdevil Mephistopheles^M, Demon Lord Nocticula^M, Demon Lord Pazuzu^M, Empyreal Lord Cernunnos^M, Great Old One Tawil At'umr^M, Horseman Charon^M, Oliphaunt of Jandelay^M (CR 30)

• Plant: Great Old One Xhamen-Dor^M (CR 26)

• Undead: Agmazar the Star Titan (kaiju) (CR 26)

GENERIC

• Aberration: Drakainia^M (CR 25), Havero^3.5 (CR 24), Conqueror Worm (CR 21)

• Animal: Great White Whale (CR 14)

• Construct: Charnel God (CR 23), Devastator^M (CR 22)

• Dragon: Elder Wyrm, Great Wyrm Planar Dragon^†, Guardian Dragon^M (CR 24)

• Fey: Glaistig^M (CR 21), Whisperer (CR 20)

• Humanoid: Eclipse Giant (CR 19)

• Magical Beast: Tempest Behemoth (CR 22)

• Monstrous Humanoid: Euryale (CR 20)

• Ooze: Oblivion (CR 20)

• Outsider: Danava (titan)^M, Hekatonkheires (titan) (CR 24)

• Plant: Green Man^M (CR 26), Mu Spore (CR 21)

• Undead: Grim Reaper (CR 22)

• Vermin: Duneshaker (CR 18)

^MMythic

^3.5Published for D&D 3.5.

^†Planar Dragons: Apocalypse, Bliss, Crypt, Edict, Havoc, Infernal, Paradise, Rift, Tumult

I've edited this post in response to some comments noting mistakes I made; originally, it was just one list, which said "no mythic, no class levels" but included unique creatures. Thank you to everyone who corrected me, and to /u/Lokotor in particular for suggesting separating the uniques.

Thank you to /u/AlleRacing, /u/zspade, /u/kitsunewarlock, /u/DMXadian, and /u/Cronax for contributing additional items and replacement items to this list!