I love the customization. The ability to make a concept, run with it, and tweak it to meet the exact idea you had when you came up with it. This makes skills and skill points one of my absolute favourite parts of the game. I really don't want them streamlined (D&D 5e I'm looking at you: lame skills system). To my mind, this is the core, the very essence of Pathfinder. That ability to turn your creative concept into something represented by the stats of the game, is what sets PF above all other games of similar design.
The setting (Golarion) is fantastic, as are the materials such as adventure paths and campaign setting books.
Some things are flexible, such as actions in combat, spell methods, etc. But the customization (skills, feats, skills, archetypes, skills, alternate classes, etc) are the very best of PF.