There are two options.

Kamen Rider Fourze and Kamen Rider OOO. Fourze has this delightful childishness throughout the series, but still delivers the feels. Has good stories throughout and some better characters. OOO delivers heavy on the story, heavy on the characters, heavy on the villains, and perfection from start to finish.

Well, it depends what period of Rider you want to showcase, because the series are so different from each other.

W, OOO and Fourze are all very good examples of modern Heisei riders, they set a trend in tone and writing that only Gaim really broke so far.

Early Heisei is a bit harder to pinpoint, because they were all pretty different. Though Kabuto and Kiva feel very representative for that era...
Blade would be fine too, but it might set the expectations in certain points too high.

As for the 90's movies, I think they're all fine, but J and ZO do feel more like typical Kamen Riders, so I'd rather go with them.

Can't say much about Showa series, because I've only seen Amazon and 4 episodes of Stronger, but I guess the Showa rider shows are very similar in style, so I guess they'd all be fine.

I think one of the best shows to represent the franchise as a whole is actually Blade. The show has some typical Heisei elements, but also has some more Showa-esque elements like the conflict of riders being kaijin (Hajime, Mutsuki), the rider's own happines being sacrificed to protect everyone (Kenzaki, Tachibana) and also a sort of heroism that I find very interesting to watch (mostly seen with Kenzaki).
OOO is very similar to Blade in some points, so I would say it's kind of like a lighter version of that, but with a less Showa-esque touch.

This is correct. Well said. :thumb:

Kuuga. Because some people don't care about Showa.

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the best ones are probably W or OOO. but you should never attempt to bring someone into the fandom through den-o, I tried that and now all my friends think that is what Kamen Rider is.

I say W is a pretty good series for showcasing what Kamen Rider is.

Depends on which time period we're talking about.

Showa: Black

Early Heisei: Agito

Mid-Heisei: Den-O or Decade (yes, I know, we all hate Decade, but it's hard to deny that both he and Den-O have become the 'faces' of this whole era, if only because they're the two Riders that have made the most noteworthy appearances after their shows ended).

Neo/Later Heisei: W. Slightly edges over OOOs just because W is the one that set the formula for later shows (minus Gaim, which, while probably being the best written show in this entire era, is too much of a deviation from the rest of the franchise).

Sometimes I think I'm the only person who doesn't hate Decade. It's flawed as all heck and really feels like the middle of a series more than anything, but I can enjoy it for what it is. In particular, I like the design aesthetic of the series and it has a hell of a Driver. It may help, in my case, that I saw Decade before any of the series reflected in the AR worlds. :shrug:

Edit: Also, Narutaki is awesome. :D

Those people need to watch Kamen Rider Black.

I did, too. I still hate it as much as possible.