Diend is more of a troll, rather than a straight up &*#$€bag, so my vote goes to Kaixa

Okay. Not knowing either one I voted for Kai. Because it looks like Diend has a file-divider glued to his costume.

Oh yeah, that episode. Made me want to beat the shit out of him, even after that one Ayakashi stole his Diend Driver. The whole Decade/Shinkenger arc, it's all his fault, anyway.

Where are Stronger and ZX? They're also red. Besides, Kabuto's design was partially inspired by Stronger.

Not really. Kabuto and Stronger share a motif. That's pretty much it...

Areaseven, the riders in the tournament are the winners of their respective threads

Kusaka is a sociopath who's main role on Faiz is to create conflict by being a complete and utter dick to everybody. The strongest example is the time when he notices that Takumi (Faiz) and Kiba (the horse Orphenoch/secondary protagonist) are becoming friends. Kusaka then steals the Faiz Gear and attacks Kiba with it so that the two will start hating each other (a problem made worse by the fact that it's Faiz and nobody on the show is physically capable of having an open and productive conversation). Kusaka is, hands down, the douchiest Rider ever. Kaito's also kind of a dick, but at least his heart is usually in the right place.

Also, Kaixa's design is just way cooler.

Kaito is more of a troll, but he's a glorious troll so he gets my vote.

You know what, I think I'm missing a few names here. What other Riders were complete douchebags in their series (or are there any outside of Kusaka and Kaito)?

Take the Ryuki cast and throw a freaking dart. If you don't hit Ryuki or Raia, then you've got a D-bag.