The built in defragmenter works just fine.
By default, it should be set to an automatic schedule to defrag when it hits the 10% threshold.
Having said that there are other 3rd party defragmenters that offer more features, are more configurable, and have various ways of defragmenting the hard drive.
If you prefer to go this route.
You will get many opinions on the subject, but my personal favorite 3rd party is Perfect Disc by Raxco and its Microsoft certified.
This is a paid program however.
Whats "best" probably comes down to user preference more than anything.
But,as I said, the built in one should work just fine.