Kamen Rider Den-O
I've just finished the series (although of the movies, I've only watched the first). The ending didn't feel like it resolved as much as I thought it was going to, and I was never able to quite wrap my head around its time travel rules. Additionally, a deeper dive into the villain's motivations would have been nice (I had a similar problem with Gaoh in the movie).
However, despite all of that, I still enjoyed the series a lot. Almost the entire cast was lovable to the end, especially the Imagin; I can see why they still show up on a regular basis 13 years later! Ryotaro and Yuto both had some great character development, with the latter's in particular leading me to like him a lot more by the end of the show than when he first showed up. The suit and monster designs were also all pretty fantastic (some of my personal faves were Den-O's Rod form, Zeronos Altair and Zero, and the spider, wasp, and armadillo Imagin).
Overall, pretty happy that I went with this as my first Rider series, as it's helped convince me to go further down the Toku tunnel. I'll probably try Kuuga or W as my next Rider series. But before then... Dogengers!