What is the best open source Torrent download software for Windows?

What is the best open source Torrent download software for Windows?

Something like uTorrent or Azurius, but open source? Which open source application is the leader?
Guys, thanks for the answers so far! However –
1. uTorrent is closed source but free.
http://www.utorrent.com/faq.php#Is_.C2.B5Torrent_open_source.3F
"Is µTorrent open source?
No. It is not likely to become open source ever. "

2. Azureus is now ‘Vuze’. Ss this open source? I don’t understand. Can anyone help?