Each person has his/her own perception of which application is better than another, but there are standard characteristics evaluated for each product type. For instance, defragmenters are evaluated for speed, fragmentation levels after fragmentation, the ability to defragment in the background/real-time, and the ability to even prevent fragmentation.
On a side note: Windows' defragmenter was originally a bare-bones copy of Diskeeper, though the two have been largely developed individually for the past decade or so, with Diskeeper being the 'advanced' third-party option.
Hope this helps,
This is something I've been curious about. How does CNET (or any other technical site) determine if a third party defrag program like Smart Defrag is better than Windows Defrag. It's not clear from reading the reviews. Not that I mean to single out Windows Defrag; I just happen to be working with it today. Thanks everyone.