My first ever legitimate Kamen Rider series is W, while it has its own major shortcomings I still enjoyed it especially Shotaro's heroic moment in episode 48 :)

Ryuki. I got interested in it through some slightly convoluted events. I got really into figmas last year, and was buying up a few random ones that simply looked cool. Very early on I got a Kamen Rider Spear, and from that I got interested in watching the original series from which the design came from. (Never watched Dragon Knight, anyway). Ryuki is still my favorite series.

my first Rider Series was Kamen Rider Ryuki but it was not a sub it was a Arabic dubbing on a TV channel called Space Toon

My first (and still maintained as my favorite) was KRW.

Kamen Rider Black although, I was aware of some of the older series.

First episode I watched: Wizard
First Series I watched all the way through: OOO

I started watching OOO after seeing the Super Hero Time ads while watching Gokaiger. Looked into a bit and was basically sold after seeing how crazy the belt was :lol

Technically my first rider show was Dragon knight, but then I looked W after that from ads during shinkenger.

Scine then ive watched
Gaim
Drive
Fourze
den-O
icigo

Its safe to say I'm hooked now.

The first time I really got into Rider was with the first episode of Decade. I watched some Kiva before that but didnt like that at all (that has never changed) and never wanted to touch the franchise again. The Rider War and everything that went down in Decade really got me into the Idea of Rider though.
Before the next episode aired I wanted to actually watch a whole show and the first suit that really caught my eye was Kabutos. The Belts sounds helped too.
So I blasted trhough that show and instantly fell in love with Rider.
The slick designs, cool sounds, great music and douchey testosterone filled characters where exactly what 17 year old me wanted to see :lol

Black for me... and after several years Ryuki brought me back to the fandom