PerfectDisk gets my vote. Diskeeper's defragmenting process might be faster than PerfectDisk's, but the quality of Diskeeper's defragmentation is horrible. I couldn't even tell if defragmenting w/Diskeeper really did anything to my computer. The only rant I have about PerfectDisk is the time it takes to defrag.
P.S. I have also heard great things about O&O Defrag..haven't personally tried it but plan on testing it when my new laptop comes in.