So
this was posted, and I'm bored.
The one that was your first: Kabuto
The one with the best theme song: Ryuki
The one that made you laugh the most: I don't know, but Ryuki has the funniest scenes to me (Shinji in jail, Shinji putting out an iamginary fire in the OreJournal offices, etc etc). Den-O is obviously much more of a comedy series as a whole, but really, those Shinji moments are hilarious.
The one that made you cry the most: Oh. Hmm. Blade. (I'm judging this by the finale, and to be fair, I was too shellshocked by Ryuki's end to even think about crying)
The one that surprised you: Agito, right around the Akatsuki-Go episode it made an unexpected turn and became so much more than I expected it to be.
The one that didn't meet your expectations: Amending this response, because the single thing in Kamen Rider where the fact that my expectations and the actual story put on screen were such different things that my ability to be objective about it was completely compromised is the last Decade movie. The caveat is that I really enjoyed Decade the series for what it was, so I'm talking especifically about the last movie.
As an overall series, it's W, because it didn't seem particularly interested in classic Rider tropes to the point where it was almost more like live-action shounen anime, and the Rider tropes are a huge part of why I like the franchise. I did actually like what I saw of it, with the exception of Akiko. Admitedly I also lost interest early on and never actually finished watching, so... yeah. :Dv
The one with the best relationship: Depends on what kind of relationship? Best bros = Ryuki (Ren & Shinji). Best ship for slashy tinhattery = Kabuto (Hoppers)*. Best canon romantic ship (as these things go in KR) = Ryuki (Kitaoka/Reiko)**. And as for most interesting relationships between the various characters... well, Faiz.
*Actually, I tend to NOT be into slash for Kamen Rider. There's only two slash ships I actively fangirl in Kamen Rider (Hoppers and Daiki/Yusuke), and a couple more that I think are cute but that I'm not that invested in (Takumi/Kiba and Kenzaki/Kotaro).
** I know that Yuuto/Airi is like this very romantic, sad, two ships passing in the night thing, so Den-O should be more of a contender... but I haven't actually FINISHED Den-O. And well, the Kitaoka/Reiko dynamic is a pairing trope I've always been a sucker for.
The one with the best fight scenes/action: Kabuto.
The one with the best suit design/transformation scenes: Kabuto overallETA: Kabuto for suit designs, but Ryuki for transformations. It's not that I think the Ryuki transformations are ~the awesomest shit ever~, necessarily, but the Mirror World element was something that caught my interest early on, so there you go. I do wish they'd developed that aspect more.
The one that you compare all the others to: Amending this response also. If the question were backwards, OOO is the one I compare to all the others, in the sense that it borrows rather liberally from a lot of Heisei series. (On the other hand, if the question is which series set the bar for me, which is not the same thing as 'what I compare all others to', then that's Ryuki).