[Magnugiga]

Oh, that's a fun question that can be answered in multiple different ways. My picks would be (spoiler'd just in case):

Black: Shadowmoon's entrance and his complete ownage of Bilgenia; the first time that iconic clicking sound was heard is a defining moment. Battle Hopper's introduction and the whole final battle sequence are also strong contenders.

Kuuga: Godai's first transformation into Red Kuuga. The first episode was already a defining one, particularly given the historical context of long KR hiatus that precedes the show.

Ryuki: the penultimate episode i.e. Shinji's death.

Faiz: story-wise, the one where Takumi's true identity is revealed. Tonal-wise, that absurdly hilarious scene where Kusaka was being a complete asshole to the grievously wounded Kiba.

Blade: Final three episodes, as the heroes realize there's no clean, happy-ending-for-everyone, solution to the Joker dilemma.

Hibiki: unfortunate, but Episode 30 is always going to be the first thing that's come up to every discussion about this series.

Decade: ... eh, I don't know, the series was a jumbled mess. Okay, best moment would be Natsumi transforming into KR Kiva-la and slaying Decade from the movie.

Gaim: the episode where Hase went berserk, it's revealed that Kouta had killed Yuya, and the introduction of New Generation Riders (I think it's all in the same episode? Such a game-changer, in any case).

Build: the reveal of Sento's true identity.