BLAM!
http://gomenrider.blogspot.com/search/label/agito

its not complete but still

I'm gonna throw in a vote for Kabuto. Dark series, not a typical main rider, tons of riders in the show. By far my favorite suit gimmick. Only thing is the sorta lackluster ending, but behind W, it's my favorite series

Look up Cruel Angel Productions, they have all of Agito subbed.

Have to second everyone that said Agito. Hits everything your looking for.

I'm starting to think I should watch more Agito.

Agito, Blade, and I'd even throw in Faiz and Kabuto to that.

Give Double about a 10 episode chance. If you not hooked by when Accel appears then it's probably not going to be your thing.

Kiva is a great show. I started off hating it & then about half way through, I really started to enjoy it. Once I got my head wrapped around the "time flipping".
Wataru takes a bit of getting use to, but the stories lines with Otoya, Jiro & Queen were awesome.
It also helps, that I was in love with Kiva's suit design's before I had even watched it.

Kiva needs more love! I think it's an underrated show.

i would have to agree with faiz, either of theze would be a great fit for you

Eh, there's dark and serious and then there's Faiz. I personally think that Faiz tried too hard to be adult and dark and ended up going way, way overboard. That show is less "dark" and more "unrelentingly bleak" in a lot of places. Plus once you realize that the entire plot hinges on the writers refusing to allow characters to resolve issues by taking ten damn seconds to explain anything the show just becomes annoying.

I would definitely agree with Blade and Agito, though, they're definitely two of the best Heisei series if you want something serious and dark.

watch out for the first half of Blade though

and I agree Faiz tried way too hard to be some super serious show, and everyone is super emo because of it. I wouldnt recommend it at all