Kill6BillionDemons II: The Successor's coming will be followed by 108 burning threads

[Professor Gnoll]

Where was it ever stated he was the best?

Meti was explicitly the strongest, and Maya was her only pupil.

My understanding is that the other Demiurges look down on Incubus because he was NOT stronger than Maya, and only acquired her Key and throne when she abandoned them (for unknown reasons that we will hopefully learn in this flashback).

I was extrapolating slightly- Abaddon says that "some would say" Incubus is the very best at Sword Law, and the alt text for a recent comic refers to Incubus as an "unparalleled master" of it (also mentioning that as he only knows how to kill and maim, he is an exceptionally poor swordsman).

So Incubus is quite possibly better at turning men into ghosts than Maya is. Of course, we just don't really know yet- though we may find out before this book ends. (As a side note, Meti isn't explicitly the strongest- Lord Intra seems to have had a similar grasp of the Principle Art, and is indeed the original, titular King of Swords.)

The point is that mechanically mastering sword law doesn't make you a good swordsman. Incubus has (presumably) learned or stolen Meti's sword techniques, but completely ignored the philosophy and deeper understanding behind them, which in K6BD makes him a terrible swordsman because he doesn't actually understand the point of the exercise and isn't capable of reaching Royalty.

It's almost certain that the alt text on this page describes him.

Pretty much this.