Well, I haven't been able to see all the rider series, so far. But out of what I have seen, this is my List-
5: Kamen Rider Gatack- Kagami Arata went through so much throughout the series. He learned to finally be himself. Once he felt comfortable with who he was, and not worried about how others felt, Tendou finally accepted him. He learned the hard way, but he became a man.
4: Kamen Rider Zeronos- Yuuto was awesome, the way he interacted with Deneb, and how he eventually warmed up to everyone else. Plus, every time he changed, it took something away from him.
3: Kamen Rider Black- First series I watched as a whole. Just an amazing series with a great story that continued over into RX and further with the SIC series.
2: Kamen Rider Garren- Personally, I think he has one of the coolest designs other that my number 1 fave. Plus, I like tragic heroes. Even though it was just a show, Tachibana-san made me appreciate the way my life turned out. IT could always be worse.
Finally. Here it comes....
1: Kamen Rider IXA- Yes, he has had many different users. But, when I think of IXA, I think of 2008's Nago Keisuke. He was badass. He believed in all the things I do. And, I love IXA's design. Save mode looks like a paladin/knight. Burst mode looks like samurai, and Rising mode is reminiscent of Gatack (to me any ways).