That's like asking me what bodily function that I value the most and would choose to live with, then taking away all everything else. In all honesty, I love a lot of them, in different ways and for different reasons. This is cheating, I know, but my top favorites in no particular order
Nigo(Ichimonji Hayato), Black RX(Minami Kotaro RX version), Gills(Ashihara Ryo), Hibiki(Hidaka Hitoshi), OOO(Hino Eiji).
For all of my picks, it's a combination of personality, character themes, backstory, presentation and performance of the actor, power sets and designs, and of course my personal bias.
For funniest Rider, I'll have to go with Gaim/Kouta. Even this early in his show, he already gave us a number of comedic gold moments both in and out of suit. That and combined with his personality and performance of the actor, the only reason I hesitated to put him in my absolute favorite list is because we're only 11 episodes into his series.
I dunno, I don't have anything specific in mind for a design motif. After Gaim's fruit motif had sold and proven to work at least in the narrative and thematic side of things(massive props to Urobutchi for incorporating the myth of the World Tree Yggdrasil and the theme of the forbidden fruit), I'm more open to new and different ideas, as long as you get good writers for them. A show can have perfect and cool themes and motifs for a superhero show(magic users and hope), and then falls flat on its face due to bad writing and execution.
Actually, after the whole speech of "Justice vs Human Freedom" in the Wizard special, I would like a future Rider show to take "justice" as its central theme, the same way as "half-boiled" and "partnership" are for W, "growing up" and "taking control of fate" are for Gaim. It doesn't have to be incorporated into the fantasy like OOO's desire and Wizard's hope, but is merely a central theme to the story and character conflict. I've noticed that in Japan, the concept of "Justice" is much too abstract and malleable, to the point of it being argued as a "to the eyes of the beholder" kind of thing. So yeah, I would like to see a superhero show like Kamen Rider tackles and discuss the concept of "Justice".