[Diamondmatter]

Hard for me to pick one best or worst of anything, but I'll try.

Ex-Aid
Honestly, I don't hate or even very much dislike anything about Ex-Aid. And I'm definitely bad at picking a favorite aspect since I like every aspect for very different reasons. I guess I'll go with the gimmicks/gadgets/designs for good and Poppy shenanigans for bad.

Ghost
And somewhat parallel to Ex-Aid I can't think of much I can get excited about in Ghost. When its designs and trinkets were new I thought they were awesome, but I guess it was just the hype effect since even the designs seem pretty average to me now. They all seem to have at least one awkward design choice that screw with them - or are hampered heavily by material quality, something which I rarely seem to see mentioned when criticizing suits.
Anyway, I suppose I'll go with the designs/gimmicks/gadgets for best again despite what I just said, and... Onari for worst?

Amazons
Frankly my favorite part is probably the suits, but the whole thing is pretty good in general. Ending is the worst part for me, hopefully season 2 can make up for it.

Rewatch
Judging by just how much I personally would want to rewatch something, probably Kabuto for best and Agito for worst, just by process of elimination, and not counting ones I haven't seen. Just from what I've heard Kiva sounds like one of the worst to rewatch though.

Suits
Out of all of them? I hate picking things from Ex-Aid because I worry that I'm just saying it out of hype, but Zombie Gamer Genm might just be my favorite suit of all time if I have to pick one. As for worst, dunno really. I think I'll say Zero Drive on principle of how insultingly boring and pointless it and its circumstances are. It's easier for me to choose on those principles than simple dislike.

Henshin belts
Gonna say Gamer Driver for best. I don't even care if I'm only saying it because it's new and cool. It's so fun.
Worst is another hard choice because of all the criteria. How boring it is, lazy design, ugly design, bad sounds, etc.
When I really think about it, the Mega Ulorder might be the closest to objectively bad since the toy's severe lack of sounds is the crux of a lot of frustrating questions in the show and in tertiary media like Ganbarizing.

Weapon
Weapons don't really wow me in Rider, unless they're new in which case I get permanently bored of them quickly. There also aren't any I can think of that I find particularly bad, just boring. On a whim I'll say the best is the Gan Gun Saber and the worst is the punchfists Knuckle Jimber Marron uses.

Bikes
Bikes don't really wow me either, but at least I'm better at judging their designs - probably the Machine Ghostriker for worst and BeatGouram for best.

Villains
I actually kind of hate the cheesy Shocker mooks and the actual MOTWs aren't great either, so my worst villain group is Shocker.
Best is probably the Undead. They're probably the most diverse and interesting in my eyes, what with the Tarantula Undead being good, the Spider possessing Leangle, and the Joker in general. Greeeds are a close second though.

Merchandise
Out of all kinds? I'll go with Gashats for the best and Sofubis for the worst.

Movie Taisen
Dunno, really, none of them stood out to me. They're all just kind of haphazardly crummy. I guess Ultimatum for best because lots of people like it, and Core for the worst since Kamen Rider Core was a pretty awful villain.
EDIT: Nevermind, I remembered Ghost x Drive. That whole thing was a horrible mess that did nothing but fuck with the canon of both shows and make sudden, stupid, contrived, boring plot conveniences to keep the story moving. Worst, for sure.

Epilogues
If it counts, Kamen Rider Eternal Returns for best. If not, then Chase Saga. As for worst, I guess Duke/Knuckle (specifically Duke). Or if we're sticking to this year, Heart/Mach (specifically Heart).

Openings
Another hard choice. I can't really differentiate my favorites (Anything Goes, Life is Showtime, EXCITE) into one over the other, but I'll go with EXCITE.
As for worst, Hibiki's. I was gonna say Ghost's on principle of being kind of boring and the visuals being pretty cheesy, but man. Hibiki's has no lyrics and the visuals are 70's-tier cheesy.