Best defragmentation tool ever is Diskeeper 2009 Professional edition (or Propremier if you can afford it). It has features lacking in most other defraggers like multiple drive defrag, system files defrag, low space defrag, low resource footprint etc. I like the automatic mode best of all: set once and never have to defrag Windows again. I have currently amassed about 2+ TB of drives in my system and Diskeeper takes care of all of them nicely in the background.
Diskeeper co-wrote the API for Win NT defrag with MS, and all other defraggers use it...they were into the defrag game right from the beginning.