Original series:
You can follow up from the first 13 and before 16-17 and watch 14-15. IIRC this is Taki's first time doing something he's known for a lot (he's really proving he's worthy of that badge) and well, it's the introduction of Rider 2 and full of legendary moments.
I want to say the arrivals and departures of villains, but they might be better received if you watched said villains run rampant for a while before they exit.
V3:
Episode 30. An exit of a villain... I think in this case I can give an exception as it still stands on it's own just great and if you were to watch previous recommendations you'd be a bit used to him anyway. Again, I use the word ''legendary''.
Episodes 50-53... ugh, I want to give in and say the last 13 episodes... 50 is another epic villain exit. After that it's the introduction of a new one and the Riderman arc. Your choice.
X:
Episodes 1-7
Episodes 8-21
Episodes 21-35 (with the movie somewhere there)
Choose your arc. Or the whole series. Or the movie. I can't... I can't recommend a single episode only. Well, maybe Starfish Hitler's or Spider Napoleon's.
Skyrider:
Episodes 15-17
Episodes 27-28
Episodes 41-47, or just 47?
If you liked a previous Rider you can pick their comeback episodes too.
Super-1:
Episode 42.
Jin Dogma are my favorite villains in all of Kamen Rider. Though maybe it's because they are what they're badly presented... villains aren't supposed to be loveable, right?
You might have heard about Super-1's second half, how it changed to a lighter, more kid-friendly tone.
What they don't tell you is that they went INSANE to do it. And it shows perfectly in this episode.
I always kind of joke that Jin Dogma's Evil Council of Dooooooom!11!!! is what the Taros would be if they were villains.
From the serious side, the first 2 episodes show that pretty well. Previous Riders had been resilient, dedicated people and martial artists... but Kazuya surpasses them all here.
WATCH THE ZX SPECIAL
Black:
16-21. Or just 18.
30, 34-35.
And again, I can't resist but say the last 10 or so.
If you're planning to watch the whole series I'll warn you that whenever you a character called Taki Ryusuke appears, it means the series is about to do something big. Unrelated to him, it's just the coincidence.
Those are all I can think of at the moment. You can just watch the movies too, though you'd have to settle for HK subs mostly, if you can even find those in the first place.