Samurai Champloo - Definately my favorite ost collection.

Best Opening, I can't choose, but the first that comes to mind is Hajime no Ippo's first and second opening.

Hajime no Ippo - Under Star

http://www.youtube.com/watch?v=yaW_iOMEN7c

Hajime no Ippo - Inner Light

http://www.youtube.com/watch?v=VG0WEPOVhdg

My fav Track from Samurai Champloo - Space Between Two Worlds

http://www.youtube.com/watch?v=yS6uWWrjf8c