Kabuto's the first Rider series I watched all the way through, so I'm very partial to it. While there's a bunch of plotting issues (uh, The Bee and its' importance or Red Shoes, for example), the things that keep me hooked on the series are CLOCK UP and Tendou being... well, Tendou. All things considered, anytime I recommend Kabuto to someone, I just tell them to watch the movie. Series wise, I'll stick with the Tsurugi arc, anything with the Hoppers, and the final two episodes. Something about the music during the final fight where Tendou, Kagami and Mishima face off (Tendou, after CASTING OFF, just calmly waiting before the battle starts is plain badass) just awesome.
Oh yeah, there's a Tak Sakaguchi extended cameo in the show that is way too much fun. Someone get that guy a long term role on a Rider show...