The best ever made is PerfectDisk 2000. It does the job better than any others.
But personaly, I run O&O Defrag (the FREEware version - I don\'t want to spend money for something windows offers me already) that works good too.
The June 17, 2003 issue of PC Magazine has a huge article devoted to utilities. In the Defragmentation section, they reviewed Diskeeper 7.0 SE Home Edition, JETDefrag, Norton Speed Disk, and O&O Defrag V4 Professional Edition and gave their Editor\'s Choice award to Diskeeper 7.0 SE Home Edition with 5 out of 5 stars.
JETDefrag: 2 stars
Norton Speed Disk: 3 stars
O&O Defrag V4 Professional Edition: 4 stars
The Article has good descriptions and discussions about each program.