After a bit of thought on this I went with Blade.*
All of the other Rider shows I feel were laudable either for doing something original with the show's production or coming up with a novel KR premise or design. A lot of the shows that don't so much work as Rider shows (Hibiki, Den-O) still weren't bad tokusatsu ideas at all. Actually, they were kind of brilliant, if executed unevenly.
Blade was a case of a syndrome I like to call All The Popular Elements. At the time Blade was made, Rider had been so many different thing that the fandom was getting fractious. Toei made the enormous mistake of trying to make a show that would somehow please all Rider fans by containing everything anyone had liked in Rider to date.
They wanted to please the Showa fans by giving them a purely superheroic character again in Kenzaki. They wanted to please Kuuga fans by having a mysterious plot with a big twist at the end. They wanted to please Agito fans by playing up the conflict between Blade and Garren against a procedural backdrop. They wanted to please the Ryuki fans by having morally ambiguous Riders and a card-based power system. They wanted to please the Faiz fans by telling a story driven by interpersonal conflict, complete with a sympathetic monster character or two (eventually).
Whenever you make a show in a fashion this calculated and formulaic, it ends up not working. It doesn't work because it's not really about anything-- you work in all the formula beats, suddenly there's no room for developed themes beyond what's been borrowed. There's nothing surprising or original your idea can suggest, because your entire point was to make something containing no originality whatsoever.
What I've observed over the years is that the shows that usually turn into big successes (or at least spectacular failures) are ones that are somehow personal to the creators. Because they're personal, there's a sense of the crew working extra-hard to make something good, something lasting. Because they're personal, these shows often end up reflecting the unique style and personality of the individuals who created it.
I never felt any sense of purpose in Blade, and while I did watch it a long time ago (and so mix up some bits of the plot) I watched more of it than I care to think about in retrospect. Blade to me felt like the year Toei took off while they trying to find creators with some big ideas they could use next time. This feeling was confirmed when I saw Hibiki announced....
* I'm sorry, Blade Dancer! We're still cool! I figure you defend the show here so often and so well, it can take a few lumps from me.