Mine was Blade. When Sayoko was murdered, it had me hooked that something like this could be compelling in its story telling. When I learned that Masked Rider was Kamen Rider Black RX, I also learned about the differences in cultural story telling.

Heh. Nice.

I played the ZO Mega CD game, does that count?

Otherwise the first complete series I watched was Kabuto.

My first rider series was Kamen rider ooo and I thoroughly enjoyed it and I still love all the characters and forms. My second was Den-O and that was just pure comedy to me with all the taros in it.

Gaim for me.
I was hooked from the minute I saw the first episode, and then I was introduced into the wonderful world of Kamen Rider.

First exposure was to Kamen Rider ZO, which lead to older series which I think were better.

Dragon Knight for me which lead to research that led my into this rabbit hole we call Kamen Rider and Super Sentai Which also spawned my discovery about the KibaRanger from Dairanger which was pretty shocking for 12 year old me

Kamen Rider OOO

And also wizard.

I started OOO at the time Wizard started, and watched both. I kinda regret watching Wizard. I don't regret watching OOO.

Wizard. It was the first series I knew of because I was watching my first Sentai, Kyoryuger, at the time and because a friend kept mentioning it. I decided to give it a go. I don't find it as terrible as seasons like Kiva, but it definitely wasn't great. Doesn't mean that I didn't enjoy the show. Had the best fights, designs, and (vocal-less) soundtrack out of all the seasons to me.

I think my first Kamen Rider was Decade. Me and my friends back then were like "Onore Decade!" or something like that and people thought we were crazy..

Fourze. And I discovered it through TJOmega on youtube.