I prefer Wiztree (link below), especially on systems with much larger disks. Since Wiztree works by scanning the MFT it's about an order of magnitude faster than Windirstat or Treesize.
Sadly as a consequence of this, it can only report file size, not disk usage. If you have a lot of folders with NTFS compression enabled, Wiztree becomes almost useless.
I like it too but I've found it to be pretty buggy sometimes, especially if the contents of a folder change or the folder itself is deleted while you're viewing it, you get an endless stream of Delphi exceptions.
http://antibody-software.com/web/software/software/wiztree-f...