5. Stronger - I'm fine with it from the neck down, honestly. The chest shield is a little puffy but fine, the belt is okay, and I actually like the way the stripe runs down the arm and onto the gloves. That said, the giant doofy helmet completely kills the look for me. Did the stunt actor have a massive afro or something that needed accomodating? I just cannot take that enormous head with all the obvious eye holes cut into the lenses seriously. Especially since I cannot look at the mouthpiece and not see it forming a happy little smiley face.
4. Climax/Chou Climax Form Den-O - I'm not a huge fan of the Den-O design aesthetics on a good day. Zeronos, New, and G Den-Os are great; I can put up with Sword, Axe, Rod, and Liner forms; Gun form is... a valiant effort. Climax form, though, dear god that's a mess. So many clashing elements on one suit, made even worse when you slap giant blue wings on the back. And again, the helmet is really terrible. The color on the lenses is neat, but overall its far too busy of a design.
3. Masked Form Sasword - Every crappy, short-lived mid-90s toyline called; they want their design back. I have never seen any design in Kamen Rider that was more obviously meant to represent a toy (not counting ridiculous vehicles from Kiva or Double). Everything about this guy - bright purple, crazy armor, green Cyclops visor, day-glo orange hoses everywhere - screams cheap, ugly action figure.
2. Rising Ultimate Kuuga - No. Just... no. Not only did this completely fail to understand how the Rising and Ultimate forms actually worked on the show, but it is one of the most ridiculously over the top designs ever. The original Ultimate suit was a classic design that took the base Kuuga form and built onto it in imposing and frightening directions. This suit barely even looks like Kuuga anymore. It's like someone chloroformed Yusuke, took his unconscious form to an LA body shop, and let them just go nuts. Horrible, horrible costume.
1. Decade Complete and Diend Complete - I'm calling this a tie since they're effectively just minor variations on the same deeply flawed concept. I don't mind the silver and black base suits, but the armor and helmets on both are just atrocious. I have no idea why anyone would ever think that the unnatural combination of a Kamen Rider and a trading card folder would make for a good design, but it's quite clear that they were dead wrong. I'll give Diend some bonus points in that the two-layered look is less clunky than Decade's shoulder-to-shoulder spread, but it's still a terrible design choice.
So that's my five. I'll toss out a couple dishonorable mentions to Shuki (a skirt, really? This isn't Sentai) and Touki (giant fake muscles look dumb on kids' Halloween costumes and they look dumb here), but neither were around long enough or egregiously bad enough (Shuki's true awfulness coming more from her message and purpose - i.e. "women aren't fit to be Riders") to make my top five.