[Suzu]

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.