Kuuga or Agito, since theyre both in the same continuaty.
Kuuga will give you some Heisei vibes, with dark storylines and kickass action. But you might find it boring compared to a sentai show because of all the exposition. Odagiri Joe gives the performance of a lifetime as Godai, leaving behind a toku fandom pleading Odagiri to return once again to Kuuga. The story of a solo rider may be a turnoff for some people used to a sentai story with 5 people and a eventual 6th rider.
Agito on the other hand is pretty much the same as Kuuga, except less bike action, and more riders. Agito has some more exposition too, but mostly with Hojou and Mana, both are integral parts of the series.
The exposition in both stories makes the story flow without plotholes with great success. Agito's finale was lackluster to some compared to Kuuga, which was a bit more gruesome, but both are great series's and are the pinnacle of the Heisei Rider Shows.
Ryuki is what alot of people start on, and the finale will most likely make you want to smack someone.
Faiz gets repetitive, but still better than Ryuki. What I liked about Faiz was the Orphenochs and the suits, they're great.
Blade is really good series. Great characters, everything is solid. Solid show, great ending.
Hibiki is a good show, atleast for the first half. The cast is all extremely talented, and the premise of the show is cool. Especially the fact that being a Oni is not a job, but a way of life, and a Oni must abide by that way of life. The second half goes downhill due to staff changes. ( Inoue kinda ruined the series and any chances of Hosokawa returning to a rider show )
Kabuto: After the mess that was Hibiki, Toei tried make up for it with Kabuto. Which to me, the premise was really good, like Hibiki's. Mizushima Hiro is great actor and portrays Tendou well. Some didnt like Tendou because he was a douche and was a very static character. Action and suits were very cool. Plot holes take it down a notch, but hey the fighting was still cool.
Den-O: The money-making machine and the show that launched Takeru Satoh's acting career. Funny show, some dont like it because it strays away from the basis of Kamen Rider, and some love it because it is funny. Takeru Satoh and Momotaros make this series roll. One thing I liked about Den-O was how it stuck to its motif very well.
Kiva, good premise again, but Inoue sorta screwed it up. Still, some like it. Otoya probably gets credit for that though
Decade: Some love it, some hate it. Alot of people hate it because it didnt cast the original riders and strayed from the original image, but thats just an opinion. Watch it for yourself
Double: Amazing series. Kiriyama and Suda make for great leads, and storytelling and execution is wonderful. Pleasure so far, and one of the best of the Heisei. Took what Kamen Rider learned the last 10 years and put it all together.