A series made by our very own Ryan Swag, the primary focus is breaking down the most realistic and useful IV builds for specific Pokemon for the Great League and/or Ultra League. Note that while some of these may end up out of date as the game changes and new Pokemon are added, but the IV spreads mentioned should remain relevant for many matchups.
Many of these guides also have companion videos, so please take a look at Ryan Swag's Youtube Channel for these, as well as additional useful Pokemon Go content.

Goodra PvP IV Deep-Dive

Clodsire PvP IV Deep-Dive

Giratina - Origin Forme PvP IV Deep-Dive

Cresselia PvP IV Deep-Dive

Annihilape PvP IV Deep-Dive

Decidueye PvP IV Deep-Dive

Giratina - Altered Forme PvP IV Deep-Dive

Quagsire PvP IV Deep-Dive

Wing Attack Charizard PvP IV Deep-Dive

Carbink PvP IV Deep-Dive

Greninja PvP IV Deep-Dive

Sneasler PvP IV Deep-Dive

Shadow Alolan Sandslash PvP IV Deep-Dive

Haxorus PvP IV Deep-Dive

Tapu Fini PvP IV Deep-Dive

Shadow Victreebel PvP IV Deep-Dive

Slowpoke Family PvP IV Deep-Dive

Chesnaught PvP IV Deep-Dive

Cobalion PvP IV Deep-Dive

Virizion PvP IV Deep-Dive

Guzzlord PvP IV Deep-Dive

Toxapex PvP IV Deep-Dive

Dubwool PvP IV Deep-Dive

Galarian Stunfisk PvP IV Deep-Dive

Malamar PvP IV Deep-Dive

Deoxys - Defense: PvP IV Deep Dive Analysis

Is Your Buzzwole Bad?

Scrafty PvP IV Deep-Dive

Obstagoon PvP IV Deep-Dive

Lickitung PvP IV Deep-Dive

Eeveelutions PvP IV Deep-Dive

Cofagrigus PvP IV Deep-Dive

Trevenant PvP IV Deep Dive

Sableye PvP IV Deep Dive

Noctowl PVP IV Deep Dive

Walrein PvP IV Deep Dive

Venusaur PvP IV Deep Dive

Shadow Nidoqueen PvP IV Deep Dive

Diggersby PvP IV Deep-Dive