If I have to go with just one, as the best, it'd be the original. Admittedly that's also my favorite series, but because I generally like all the Riders, what sets the original apart as the best one is also of the many reasons it is my favorite.
What the original series really has is a total clarity of concept. Every Rider series since then seems to go through a phase where the show has thrown a bunch of things against the wall, and then it decides which ones it's going to keep and which ones it'll ditch. Sometimes it's right near the beginning, sometimes it's later on, and sometimes it's a continuous series-spanning thing. V3 spends about 12 episodes trying to figure out how to top Doktor G, in the process creating two of Rider's best gimmick villains ever, a bunch of remakes of original series episodes, a not-Taki guy, some kickass team-ups with the Double Riders and finally Riderman. At some point Ryuki goes from being about Riders fighting and dying to what the guy who has them all fighting is really after. Decade has our hero travel through the 9 Rider worlds, dick around for a bit in the Inoueverse, then team-up with Amazon, Kurata Tetsuo and a Sentai team before finally going to his non-ending. I love all that of course, but it does feel like there's always a time when the writers turn to each other and go "Hey, I just had an idea."
The original does this, but probably because it does it for the first time, it somehow feels more organic. The shows's own evolution is part of the storyline. It does things no other series has done since out of necessity, and it also does things every series have done since as convention, though the first time around it was all fresh and untested. It could have screwed up on so many things, yet it gets them all right. It hit the ground running and leaped over every obstacle in its way, finally coming to a stop as the masterpiece that is the ending (ironically, my favorite episode of Kamen Rider is the last one!)
The story is simple, but effective. While not the Machiavellian labyrinth some other Rider story lines are, it's got enough shake-ups to keep it interesting. There are some good ongoing stories, though basically every episode can be enjoyed on its own. There's a pacing to it that always gets in enough action and plot, however streamlined. There's a horror aspect to the show that I think is evident throughout the series (not just the early episodes) but its always kept in check. Things are never too grim nor too light. It's just the right balance of fun and serious storytelling.
The characters are wonderful. Despite the heavily rotating cast, the show makes good with what it has. Hongo is awesome, though I think he actually gets better later on once Ichimonji has been around to expand upon the Rider concept a bit. Fujioka's able to put a bit more of himself into the role, coming across as more assertive (the clothes help too. Later-series Hongo has some of the best clothes of any Rider, ever.) Ichimonji is so badass it should be illegal, and I think really defined a lot of what makes Kamen Riders today. The episodes where they're onscreen together feel like a real meeting of titans.
Taki is in my opinion the show's actual main character, along with Tachibana and the Shocker Leader to a degree, because he's around through all the changes. We're with him 'til the end. He rocks, and I love that he never becomes a Rider. That would have cheapened the character and what he represents (the Spirits thing is okay because he's still not really becoming a Rider, just a facsimile.) While the show goes through umpteen million sidekicks, there are some great ones. Ruriko, despite her brevity, is excellent; as is Yuri, who turns up in 14 and lasts until the end. She practically ties with Taki as a main character! And I got a soft spot for Choko, who by her name alone produces one of the show's best running jokes.
And then there's the villains. The monster of the week are all memorable, with quite a few being the DNA templates for every similar monster since. The main bad guys are iconic as hell, brought to life by little more than some great acting. Shocker feels like a true worldwide menace, more far-reaching and powerful than any Rider villain group since, even if that isn't supposed to be the case! Gelshocker too, which I consider to be sort of a limited-time only Holiday Edition with new packaging, but the same great taste.
The designs and costumes are phenomenal. While costume-making has improved and the designs have gotten more intricate, there's something about the roughness of the old stuff that makes it the most impressive. And as everybody knows by now, I think the best Rider costumes ever came out of this show after episode 52. :laugh
As for effects, I would have to care about effect quality for those to even matter. But I kinda don't, so there you go. Speaking from a purely technical standpoint though, while they are the most primitive, I think the effects in the original series really are rather good for the purpose they serve. I like the trippy death scenes and the quick-cut models. Honestly it looks no better or worse to me than the CGI we get now, which looks exactly like CGI, but who cares? It's about what the effect is doing for the story, not the other way around.
The music is incredible, both the songs and background stuff. The original series songs are so good that I think you could set footage of any other Riders to them and they'd still fit. Particularly "Lonely Kamen Rider"... well aside from that one mention of the Cyclone!
I can go on about this show forever. It may be the first, but I think it's also still the best. Every Rider series brings something new to the table, and thanks to the original, it's a very big table.
