[Kiwami]

With Den-O it all boils down to if you like the Imagin or not. And it's enough to watch the first 5-ish episodes to realize that. You love the Imagin? You'll usually be entertained enough to ignore most flaws the show has. You hate them? You will stop watching the show.

So if you like the look of Den-O, try it.

Aside from that, I personally enjoyed most of the pre-Decade shows. Kuuga is good, Agito is good, Ryuki’s good, Kabuto is good. I hate Faiz and I occasionally argue with myself if I have the strength to watch that terrible show just to be able to say that I watched it, but I can't recommend this show, it sucks.

Blade I want to watch next and Hibiki sounds cool too, but the second half is trash because of issues with the producers.

Kiva I'm not sure what to think about, but I haven’t watched it so I can't say watch it or stay away from it, I don’t know myself yet.

So as far as my opinion goes:

Kuuga: Yup.
Agito: Sure (but maybe wait and watch it after Kuuga)
Ryuki: Absolutely, fantastic show.
Faiz: Nope. Don't.
Kabuto: Yeah it's cool
Den-O: Try it

So I would say try out Ryuki, I think it's the best pre-Decade show. At least the best I've watched so far.