I'm getting the bad stuff out of the way first.
Decade - I hate the ending so much I made up a different one in my mind. The Tsukasa/Decade we know is actually an imperfect copy or clone of some sort of the main villain, Dark Decade the destroyer of worlds. In the end
Kamen Rider Decade teams up with the Heisei Riders in a final showdown with Dark Decade and put him down once and for all.
Narutaki would have been right all along that Decade is the destroyer of worlds, he just got the wrong one. It explains Tsukasa not having any memories at the beginning of the show; just having been born, he doesn't have any memories to remember. It explains Tsukasa's personality; being a "offshoot" of the original, while he's a dick he's not evil like the original one and had a capacity for good. Also, IT MEANS DECADE HAD A MAIN VILLAIN OF HIS OWN!!!
Now how hard was that? That said, watching Kamen Rider Decade during its run were some of the most fun I had watching a Kamen Rider show.
Hibiki - Though some parts irked me, I was overall pretty neutral on Hibiki's ending. And that's the problem. "It's ended, huh?" I said out loud, and just shrugged. Which is a terrible shame because I really enjoyed most of the series.
Kiva - not a terrible ending but the last length of the series was so agonizingly bad.
Kabuto - It was mostly a let down and waste of potential(Hiyori, being built up as one of the most important characters in the series, spent the least few episodes cooking and ignorant of everything that's happening. And don't get me started on how they clumsily
attempt to explain away the Red Shoes System. Or how the Hoppers were wrote out of the story after being treated horribly by the writing). The ending itself was not
too bad, just kinda "meh".
Best endings
Black - Epic. Emotional. Bittersweet. Tragic. Heroic. And still I don't think I did the ending justice. I was depressed for weeks(okay, maybe just two) after the final episode.
Kuuga - Kuuga's formula is a very unique among Rider shows, and the last two episodes were the perfect end-cap for that formula. That last fight goes down as one of my absolute favorite fight scenes in Kamen Rider, not just because of the awesome choreography and filming, but the stakes behind it, the emotions, the tragedy, and the underlying subtext of the scene.
W - A lot of people came down on the final episode for "bringing back Phillip". They don't talk about how it brought closure to Wakana's personal growth and the fate of the Sonozaki family, expanding Shoutaro's character just a little bit more, and put forth its own interpretation of the notion "hard-boiled", which I for the most part agreed to. Wakana went through the entire series as a puppet, used and manipulated by everyone around her. Even when she does it willingly, she's still to some degree manipulated. The last episode is her taking control of her own mind and making a decision on her own, with the only help is Shroud telling her to do what she thinks is right. And she decided to use all her godly powers to recreate Phillip's body. It's not some cheap fake-out. The Gaia Impact, which has the potential to turn every human being on earth into Dopants, is used for one purpose; recreate Phillip's body.
I wholly admit it's not perfect. The last episode was really rushed, and I had to watch it twice to get everything. But I just loved what it wanted to convey. Besides I really think the ending works perfectly as an epilogue chapter of a novel.
The rest are in-between, with Agito, OOO, and Blade as the better ones. And I've only watched Black and Black RX for the Showa era. Black RX's ending was great, but not the best.
It gets hate because it was awful. They shoe horned in Kiba as the final villain so they could have a final battle. There was no resolution AT ALL in faiz. I find it really funny that you can't see that.
I wouldn't say it's perfect but I don't think it's as horrible as you made it out to be. Kiba went through so much **** and still believed in the good in people, then had everything thrown in his face yet again, when Yuka was killed
by humans and that finally pushed him over the edge. He was a mental wreck when Takumi kicked his ass and roused his ideals once again. Finally, in a moment of "F it, I don't care about anything anymore", he tried to believe again and fought alongside Takumi, and died a hero.
In my opinion, he acted very human and illustrated the very 'real' clash of one's ideals and the realities of the world around him. At one point he turned on his ideals, and then sought to regain them back. It showed him as a very conflicted, and human, character.