I am still new to the journey through the decades we call Kamen Rider, but my favorite so far is Decade. I relate to Tsukasa in so many deep, feels-y ways. The most notable of which being: Most of my recent years were spent trying to find a world I belonged to, trying to find who I am and what I stand for. Izumi was responsible for showing Kamen Rider to me, and it's, with no exaggeration, literally changed my life for the best!
The first I saw was technically Gaim, but I only watched the first episode and had no idea what was going on, much less what Kamen Rider was. My first actual completed series was Kamen Rider Decade, at Izumi's recommendation.(Though I started Drive first, but I'm not finished.)