I've been deep in the Kamen Rider hole after taking a blind chance on it and it's been one of the most rewarding discoveries I've had, so I'm doing a megapost where I try to convey what's so interesting and unique about this series that you can literally just start anywhere. There's a Kamen Rider series for everyone, and I hope to convince you of it and point to where you can give it a try for yourself. Click on the read more to see more!
I've watched (most of) the original series and all of Zero-One, Black Sun, and Kuuga, and I will add that I can't recommend Kuuga highly enough. Loved it all throughout. It was the first series after a long Kamen Rider dormant period and they really brought their A-game to bring it back to life (and I don't think there's been a year without a Kamen Rider since). Kuuga is all up for free streaming on Shout Factory, as is the original and Zero-One.
I had a great time with Zero-One as well, definitely a pretty light Kamen Rider compared to some but it can still get kind of dark and sad at times. As for the original series, I would definitely recommend watching some of it but I would definitely not recommend watching all of it. If the monster in the thumbnail looks cool, give it whirl, but so many episodes are the same pattern and there's essentially no overarching narrative. Chameleon Man is definitely a good pick, I also would recommend the first two episodes for when the show briefly thought it might have an arc plot, the Cobra Man episodes because as far as monsters go he's such a loser that sucks and I enjoyed it, the Pirasaurus episodes because I'm a sucker for wrestling.
Black Sun is another one of those limited "for adults" series like Amazons and while it is very dark and violent, after watching Kuuga I think it's darkness and violence is less out of step with "mainstream" Kamen Rider than I initially thought! I had a good time with it even if it got a bit grim for my tastes, and I enjoyed having a grizzled old man rider.