Ravnican Warlock Patrons

Xanathar's Guide to Everything*

Sword Coast's Adventurer's Guide**

Unearthed Arcana^

Reasonings

Niv-Mizzet is a Great Old One due to his great age and elder intellect, and is a Hexblade by virtue of the Guildpact.

Ciszarim is a Hexblade due to his transformation into a sentient chestplate.

Azor I is a Hexblade because he sacrificed his planeswalker spark to the Immortal Sun, which becomes your patron instead.

Lazav is a Great Old One due to his odd abilities that are rare even amongst the populous of Ravnica.

Planeswalkers are all Seekers due to Seekers being, in the Forgotten Realms at least, powerful beings who can travel through the Astral Plane. Which I am comparing to the Blind Eternities.