Decade's is easily my favorite for its unique latin and chinese sounding beats and just being darn fun to listen to most of the time, and its sorta heavier techno sounding beats. Hibiki's would be a close second. Randomly, these are my favorite Decade tracks:
OST 1 track 3
[hide]http://www.youtube.com/watch?v=2PYEXuntwtI[/hide]
OST 1 track 4
[hide]http://www.youtube.com/watch?v=9CotWlpZRj4[/hide]
OST 1 track 7
[hide]http://www.youtube.com/watch?v=jiYxSdP55cE[/hide]
OST 1 track 15 - This was my favorite track from the entire series, I absolutely loved it when I first heard it during the second Ryuki world episode, really made me anticipate the release so much.
[hide]http://www.youtube.com/watch?v=IBdq889hH1A[/hide]
The one problem I do think that most Kamen Rider series have with their music is that...they have really bad musical cues. The pieces are introduced so randomly and without much work up, so it's jarring when watching episodes, I think this was most obvious with the Kabuto OST.