The Diagon War Arc in Ben 10: Ultimate Alien is the most detailed and well-structured story arc of all the story arcs in the Ben 10 Franchise. This is for a few reasons:

Complex Story Development: This is not an indiscriminate barrage of action. The discovery and plans increase intelligently, with twists that are layered, and strategies planned by Diagon. The way Diagon develops his plan over time explains how much thought process went into this story arc. You can feel it develop.

Mature Themes & Character Development: We can see Ben maturing and becoming a better hero, who is expected to take serious decisions. Likewise, we can see Gwen & Kevin develop into more than sidekicks. The emotional journey of these characters is written exceptionally well.

High Stakes and Long-Term Impact: The Diagon Arc is not an episodic series of battles. The impact of the Diagon Arc has consequences on Ben and his team; therefore, they have accountability. Every battle means something, there is real impact.

Established Extra-Dimensional Villain: Unlike other villains that appear in the Ben 10 Universe, Diagon is powerful, and he acts as a villain with purpose. He manipulates and strategizes with awareness and power. His intentions are clear cut, and purposeful. He does not panic, and is calculated in his involvement.

Overall, the Diagon War Arc is the best the Ben 10 franchise has produced in terms of story structure, character arcs, and thematic development in storytelling. This is not just a run-of-the-mill action adventure story.