Well, in terms of free defraggers, there's not much to choose from, and in my experience in testing them, they're not that great, and you're better off sticking with the Windows-integrated program.
That said, in terms of 3rd party defrag programs, I wholeheartedly recommend
PerfectDisk. I've used nearly all of the 3rd party programs out there at some point, and I've tried recent versions of Diskkeeper and O&O Defrag, and I keep going back to PerfectDisk.
I don't mean to sound like an ad for PerfectDisk here, but that's my strong recommendation.