The power defragmenter and contig are great, there are times when I feel it is quicker to defrag a partition with PD than it is to analyse the partition with windows defrag
Best of all it is free
Since I found RejZoR's PD a couple of years now I believe, I haven't used anything else. I used Diskeeper many years ago, I guess that has changed considerably since then.
I also use PageDefrag from the same author as contig, Mark Russinovich and a number of his other tools, Process Explorer, TCPView, etc.