We all knew a thread like this was eventually coming.
1. Kamen Rider Hibiki
2. Kamen Rider Agito
3. Kamen Rider Den-O
4. Kamen Rider Kuuga
5. Kamen Rider Blade
6. Kamen Rider Ryuki
7. Kamen Rider 555
8. Kamen Rider Kabuto
That's how I would rank the series currently, after Den-O ended I went back and rewatched a lot of series and found myself liking certain series more and certain series less.
I guess I'm one of the few(?) people out there that's always had a soft spot for Hibiki. Yeah, I found Asumu to be pretty damn annoying at times, but he was a kid totally bewildered by the situation he was thrown in and trying to keep up with someone he could finally look up to as a father figure. I loved the interactions between the Takeshi organization and even though the second half was pretty weak, especially around the time the writers changed, I felt they managed to handle the story pretty week, especially during the Shuki arc and that final Zanki arc. I don't really see the ending as totally disappointing, maybe not what I would have expected, but I thought it was a pretty fitting ending. Plus, the series also has the best movie to date.
I'd probably rank Agito up as first, but I liked the characters so much more in Hibiki, which is why it's not just my favorite Kamen Rider series, but my favorite tokusatsu series. The whole idea of the Akatsuki-gou and the reason behind the Unknown attacks was an amazing idea. Agito did also feel like it dragged so darn much at times, and unlike in Hibiki during the period where I felt it was better off as the "That show about some kid and guys who fight demons on the side", I always enjoyed the characters....
Which leads to Den-O. Despite the amazingly weak story in the first half, and Kai's overall motifs and history, I loved the characters second only to the Hibiki cast. I only rank it third and not second because Agito had a much stronger story structure. So despite its failings, I always thought Yuuto had an amazing history, especially with the source of his power and the characters really carried the show through some of the duller times. I also think it has the second best lead actor in any of the Kamen Rider shows I've seen.