The best is the "glue cube." Contrary to common thought, glue actually speeds up the cube by getting heated by the friction of the cubes rubbing together. Heated glue will make the cube really smooth because it is so doesn't resist flow. All you do is get a storebought cube and apply Elmer's glue.
No, but really. There is no "best" speedcube. It's all preference. Read the link the guy above gave. You can order cubes from cube for you (google it) or 9spuzzles, and etc.
Also:
http://www.youtube.com/watch?v=RlhAxLhqM7w