credit to Koryuu of TV Nihon
Den-O is my favorite. (Well, it was my first Kamen Rider so technically it would be my favorite right now. XD) It had such memorable characters that I cried in the end.
Series had a great cast IMO. Sato Takeru was a new actor (I believe) and he perfectly portrayed 7 characters, including his character, in the show. I don't know who/how someone can dish out 7 characters in the same show and continue to the end, but Takeru did it.
Here's a link to view this awesome Kamen Rider show. Link
If you want to download it for yourself, please visit TV Nihon for direct download in the forums.