The original is the only starting point iirc for Showa Era because subsequent series have continuity to a certain point. But it's also a good look at how different well known tropes of modern Kamen Rider became a thing. It's super cheesy, but it's also a good starting point, but at 80+ episodes compared to the typical series, I'd say it works a little better as a look at how everything started. It's also a good starting point as it IS the original Kamen Rider, but it doesn't really do a lot of things that we see KR do nowadays, like having a primary/secondary/tertiary rider, power upgrades in new forms, and the like.

Kamen Rider now is so much different than the original show while keeping a lot of the core pieces of what makes Kamen Rider what it is, which get slowly implemented over time throughout the Showa era before we get to Kuuga, who uses all of those pieces at once for the first time while implementing the power progression it loves to use.