Poor J? I'm pretty sure it's poor ZO. I think Doras has more appearances than ZO at this point :p

Decade's had more ambition and its flaws were more pronounced. Wizard was just boring.

The original pretty much is the best way to go, especially when read with the Ishinomori manga, which gives further social and political background and establishes what Rider exactly is fighting for and against.
Its hard to argue against the original that set the standard for everything.

After that its pretty much either Shin and ZO or Kuuga/Agito.

Shin and ZO were Riders of the 90s and establish two ideas of it. Shin does the shady people in charge and body horror and tragedy of Kamen Rider and the idea of him being a monster who fights other monsters, and ZO the idea of Kamen Rider as the hero of nature.
Kuuga of course modernized how Rider shows work and how characters work, with Agito going even further while also really cementing the anti-authoritarian ideals of Ishinomori as well as his themes of new humanity and evolution. Not to mention how it still has not one but two tragic loner of horrible circumstances as the monstrous heroes.

I'll be honest, I prefer Black RX. I keep meaning to revisit Black, but there were times it felt like a real grind to watch until Shadowmoon finally shows up. The opening episode of Black is superb though. Just wish most of the rest of the show was like that.

I know that's not the "cool" answer but Black RX just held my attention better throughout.