Levistus's_Leviathan
5e Freelancer
Wait, is that the first CR 27 monster in D&D 5e? If so, the only CR that hasn't gotten an official monster for it yet is CR 29 (the Demon Overlords from Eberron: Rising from the Last War are CR 28, while Tiamat and the Tarrasque are CR 30).So Baalzebul is CR 26, Bhaal has two forms (the Slayer and Ravager) which are CR 20 and 24, Cryonax is CR 22, Mephistopheles is CR 27, Pazuzu is CR 25, and Ssendam is CR 23. All have lairs, lair actions, and a full-page statblock.
For comparison, Orcus, Zariel, and Demogorgon are all CR 26. So these are some really, really dangerous enemies.
EDIT: Plus, they each get a full page art piece.
Another statted-out deity is nice. It's also nice that Bhaal is higher CR than Auril from Frostmaiden is. Gods should actually be powerful. (Also, lorewise, isn't Bhaal not even a true god anymore? @Henadic Theologian, do you know? You're the FR expert here. I remember reading somewhere that the Dead Three lost their Divinity recently.)
It's also interesting that Cryonax is one CR lower than Maegera the Dawn Titan (from Storm King's Thunder). I sure hope that these monster stats are reprinted in Mordenkainen Presents: Monsters of the Multiverse. (I don't have the money for this right now, unfortunately.