There are many in the series, but the final battle is the most awesome.
The Kaisers stopping Mt. Fuji from erupting certainly counts.
Dino Geist's debut. Not only is his plan legitimately brilliant and only fails because Exkaiser realizing Kouta's communicator being out was a red flag, he overpowers the Kaisers and even triple teaming him they can't so much as scratch him, only make him feel killing them isn't worth the effort with his deal ruined. Even a direct hit from Exkaiser's Thunder Flash fails to hurt him!
Dino Geist catching Sky Max alone...and completely overpowering him with one hand.
Tokuda of all people getting a chance to shine in episode 27. The Sweepers, satellites protecting the planet from meteors, are being targeted by Dino Geist, who breaks the communications relay with them. Tokuda ends up sucked into space (thankfully in a suit)...and has to reconnect the relay. He succeeds.
King Loader and Dragon Jet combining with Exkaiser to form Great Exkaiser is pretty damn awesome.
The debut of Great Exkaiser's version of Thunder Flash in same episode is incredibly epic, backed by a triumphant theme which from this point on signifies that something or someone is gonna get wrecked once Great Exkaiser is on the scene.
Episode 35: The humans finally decided to honor Exkaiser by doing a concert for the Kaisers. Of course, the Geisters had to screw it, but this is where the awesome shines: Kouta gets into the Monster of the Week made from the concert hall in order to save the trapped Kotomi, and then she, as the chosen pianist, was encouraged to play the piano while Drill Max holds the place together, the song gave strength for the Kaisers to fight on, and finally... the song opens up the concert hall's ceilings (it was designed to do that). And to top it all off, the conductor decided that The Show Must Go On, and he got all the musicians to play all the usual instruments that accompanies Exkaiser's Transformation Sequence and the summoning of Kaiser Sword. It's as awesome as it sounds, and it shows one time that the humans kind of 'teamed up' with the Kaisers to drive away the Geisters.
Episode 40 sees Kouta, trapped in the Geisters' base, keeping his head and managing to turn off their signal jammer...resulting in the Kaisers Storming the Castle. The resulting fight sees both sides going all out in a huge fight that's so intense it causes the volcano to erupt. Kouta gets the big one, though, as because of him the Geisters' Super Villain Lair has been destroyed.
Tokuda figuring out the flaw to the Lifeforce Capture Device and exploiting it twice to save the Kaisers from it.
Kouta diving into a lake to reach the Lifeforce Capture Device and free Ultra Raker and God Max. By doing so, he turned what looked like a sure win for the Geisters into their final defeat.
Exkaiser not only saving a rocket from the geisters but Launching it himself, seconds before it would have missed it's exit window!!
in same episode is incredibly epic, backed by a triumphant theme which from this point on signifies that something or someone is gonna get wrecked once Great Exkaiser is on the scene.