The first fight Kamen Rider 1 has with Spider Man and the Shocker foot soldiers. Really got to hand it to the stunt work there, especially for a first episode.
Episode 2's night fights against Bat Man and Shocker's combatants.
Episodes 6 and 7. Full stop.
To elaborate, these episodes were the first two-parter episodes in Kamen Rider history, and it delves on the MOTW Grimreaper Chameleon searching for the Nazi's hidden treasure in Japan. Of course Hongo found out of Shocker's plot and plans to stop them. Meanwhile, the Great Leader had his scientists find a way to stop Kamen Rider, preventing him to transfrom by confining him to a traphole (where he transformed anyway). The next episode, we're shown Hongo as the Kamen Rider about to be crushed by a descending ceiling, the pressure and lack of wind to activate the Typhoon's turbine resulted in Hongo reverting back to a human form. Fortunately, a Nazi box Hongo acquired from the last episode, in Shocker's hands, was a fake and he told them that only he knows where the real deal was, resulting in Hongo getting out alive and escaping. In the climax, Shocker found the location of the buried treasure and when they opened it... Kamen Rider was inside! Regardless of how Hongo does that, he fought all of the Shocker goons and defeated Chameleon. But after Rider Kicking him, instead of letting him dissolve like the previous MOTW, he Rider Chops him and cracks his skull open! Yeah, the effect was done by smashing a mirror with the Chameleon suit reflected on it, but its implication of how strong the Rider Chop could do against an enemy was quite clear!
Episode 12: Two scientists are strong-armed into making a weapon for Shocker. In the climax of the episode, the Monster of the Week is giving Hongo a lot of trouble. The scientists turn their weapon on the monster, taking him out. It's a very rare instance of someone other than a Rider, without any powers at all, just the tools at hand, defeating a monster. You can probably count on one hand the number of times it's happened in the franchise's 45+ years.
Episode 13, where Kamen Rider #1 beats up EVERY SINGLE ONE of Shocker's monsters that appeared thus far, plus the new MOTW. There's a reason this episode was adapted into a movie.
While filming for episode 10, Hiroshi Fujoka got into a motorcycle accident - footage prior to the crash was used in the episode. (He apparently remembers up to the point where he took his leg, badly twisted out of position, and realigned it. That is definitely a CMOA for the actor!) This put Rider #1 out of commission, while Rider #2 took over beating the crap out of Shocker. Several months later, Fujioka recovered and returned to the show - and thus the Double Riders were born.
Episode 15, Hayato's debut. All of it.
The awesome final showdown between Pirasaurus and Kamen Rider #2 in episode 17. Even to this day the fight choreography holds up pretty well, especially for it's time.
One from Kamen Rider vs. Ambassador Hell: Kamen Rider tangling wth the Shocker Combatmen... on horseback.
Episode 79: Ambassador Hell's wily Batman Gambit to beat Hongo, Which consists of...
Intentionally jumping the gun on Shocker's latest plan, by using his secret monster form, Garagaranda, let himself get marked for execution - wherein the Great Leader has Hongo and Taki invited to witness it.
Then Hongo and Taki save his life, for information and of course due to Hongo's pity to his adversary - which they do pull off sucessfully. Then, after he's settled down with Tachibana and the others, he double-crosses and kidnaps them.
Episode 94, where the Double Riders fight the Shocker Riders, their evil copies, and defeat them by forcing them to give chase in circles, then leaping high into the air, as their doppelgangers follow, and they subsequently collide and are blasted into smithereens.
The last episode. Where do we begin...
At the start: Hongo is frozen and cannot transform into Rider #1, helpless as Hiruchameleon is about to suck the blood off his friends... then Hayato comes in and in true badass fashion, wrecks the whole joint, allowing Hongo and the rest to escape.
Then after that the Double Riders fight Hiruchamelon and his Gel-Shocker Combatmen... on a moving rollercoaster.
And the final battle, where, in an almost the same fashion as Episode 13's, just before facing the Great Leader of Shocker, the Big Bad himself, Hongo and Hayato go toe-to-toe against an army of Gel-Shocker's monsters.
For the franchise as a whole, going a legitimate 1,000 episodes and counting. For a kid's show franchise to last that long, you know Kamen Rider as a whole is something special.
Heck, the original series lasting for 98 episodes alone is something very worthy of awe, especially when most Toku shows only ever last half as long. Especially after losing their main star very early on in production, and then making the very risky gamble of introducing a new charactrer, making another iconic hero in the process.
As a bonus, the show was slated to end at 100 episodes, before the last two episodes were reformatted to become the opening episodes of the sequel series. The show could have gone as far as the producers would have wanted to - and the endless list of follow ups practically mean that they did do so.
As a general example, each new transformation across the franchise is very likely to be one of these. The flashy special effects of the later instalments is just the cherry on the already tasty cake.