The show has undoubtedly one of the more "CGI-to-death" fight scenes. Nevertheless, one must not discredit the very first battle where Takumi, forced to wield the Faiz Gear, simply does a side kick on the first Monster of the Week and it explodes.
Episode 4. Yuka's epic revenge against the Alpha Bitches who made her life hell.
This one needs context. Kaido, Kiba's friend, had an accident that broke his hand and rendered him unable to play his guitar anymore. Yuka finds out that his professor had a hand in the accident, rigging his motorcycle so that the brakes won't turn on. Kiba goes and confronts the professor, who coincidentally is also an Orphnoch, and the two duke it out all while Kaido plays the guitar to Yuka one last time. What Kiba says to the professor before his transformation is also epic.
Of course, try creating a hovering, rocket-powered bike without it! Delta's debut, when he breaks out the Jet Sliger against Faiz and Kaixa, has to be seen to be believed. The Delta powers are generally more awesome when in the hands of bad guys, in fact.
The Movie, Keitaro's short-lived but awesome stint as a Rider.
Episode 40. Good god Episode 40. The "triple henshin" at the end remains one of the most awesome moments not of the series, but of the entire Heisei era!
This one takes context. The Riders of this series are very much not on the same page. On top of that, Takumi himself is suffering from a Heroic BSOD around this time. So he officially finally gets himself through it as he and the others put their differences and own agendas aside, and for the first time ever, the three Riders stand together, enter their transformation codes, and power up to do battle. It was quite a moment.
The theme song starts up as Takumi declares his intent to fight as a human, as Faiz. EPIC.
Also, the zoom on Takumi's hand flick. That flick made this troper shiver in awesomeness.
Faiz Vs. Orga was simply epic, as the brawl was fought in both their Rider Forms and their Orphnoch forms, and one of the bigger moments is when Faiz rider kicks Orga's sword, now charged into a Big FreakingLaser Blade, and it causes a freaking large shockwave that cuts the entire roof of the stadium and even some Orphnochs while they're at it!
Don't forget Faiz vs Psyga. A high speed chase using Faiz Axel Mode ending up in that stadium in a very exciting battle. And that finisher. MY GOD, THAT FINISHER!