I'm just getting my feet wet in Toku, and am finishing up watching W as my first Rider series. I've really enjoyed it (thanks for the recommendation Shadowbreaker!), but haven't decided where to go next. I'm leaning towards OOO before I go back and start picking up older series.
Just finished all of Ryuki last night. Wow, what a show. The ending was a bit unsatisfying though, and the footage that plays during the credits is a bit confusing but I think I know what happened. I prefer the TV series ending to the movie. A great show, but for me it trips and falls right at the end.