[MaskedRiderAsakura]

Personally for me, Agito's first appearance, Ryuki going Survive Mode for the first time, Takumi proclaiming he'll fight on humanity's side then henshins into Faiz alongside Kaixa and Delta and Gaim vs Baron final battle.

[Matrixbeast]

In no particular order:
Hyper Kabuto's debut
All Blade Riders transforming together
Blade King form's debut
Chalice going Wild
Ankh's final words to Eiji
Garren killing the Peacock Undead (SAYOKOOOOOOOO!!!!)
Kureshima disregarding ToqOh

[TheXrosLoaderGen]

One of my favorite's would have to be the final battle between Gaim and Baron and when Ryoutarou reunited with his Imagin and gained the Den-Liner Form.

[SanoBaron]

Hikawa punching Hojo into the car. rock hard manliness right there.

[CybermanWorf]

Some of my favourite moments of which there are probably more and probably ones I preferred over these from the series these are just one that I can remember from the top of my head.

Ryuki first using Survive form
Accel trial's debut battle
The final battle in OOO
Fourze mastering Fire states
The riders in Blade luring out the undead which could mimic them

[TheXrosLoaderGen]

Also, one of my other favorite Rider moments would have to be Jounouchi's Rider Kick, Drive's debut battle, and Zerono's battle debut.

[SanoBaron]

;-; Shotaro at the end of the episode of W nearly all of us cry at....

[XW1n5t0nX]

The entirety of episode 48 of W
Kouta and Kaito's final battle
The final battle of Paradise Lost
Blade's snow battles
Terui's test to gain the Trial Memory
Shin's transformation
Odin appears before the remaining riders
Kotaro's first henshin into Black
Gill's first battle
Takumi, Kusaka, and Shuji's group henshin
Shinji activating Survive Form for the first time

[Kiryu]

Final Attack RIde Hibiki:

Nuff said.

[Phantom Stranger]

Let's see...off the top of my head:

Battle Hopper's sacrifice(which honestly is a bit cheapened given the nonchalant rebirth as Acrobatter, but still.)
The birth of RoboRider, just because of Kurata's amazing scream.
Whale mutant. Just Whale mutant.
First appearance of those hilarious looking Black Satan bunn- I mean owl mooks.
Kachidoki Arms' first appearance and following rampage.
Amazon's final battle. (That includes the victory dance afterwards.)
Shadow Moon basically telling the Creation King to shut it.
Pretty much all the practical effects for Shin.
I can't pick just one scene, so I'm going to say anytime Kivat is a dry witted smartass.