Golarion's Most Powerful Spellcasters

Proud of your wizard, sorcerer's, cleric's, or bard's magic prowess. Good. Surely you've worked hard on developing your PC. But the question must arise: How does your PC stack up to Golarion's most powerful spellcasters?

I'm looking at Inner Sea Magic and to quote:

"If an NPC's level is followed by a plus sign (+), it should be considered a minimum level - that character's actual level and power are beyond that even of a 20th level character in some way, and they may even rival the power of a god."

Wow.

So who are these 20+ level spellcasters?