For actually most iconic arc in Toaru, it’s probably NT9-10. Othinus really became a fan favorite with these volumes and the most iconic villain in the series. Imaginary Fest advertised these parts like crazy with events based on them because of their popularity too
Railgun is easily its version of the Daihasei Festival arc
Everyone involved in the story in some way, great fights, Misaki technically makes her debut here, and nothing else in Railgun really topped Touma and Gunha teaming up to stop Level 6 Misaka