Answer:
I have tested over 20 bittorrent programs and they all run at about the same speeds when properly set up.
I would rate Vuze and uTorrent as the two best. They both have all the basic features that are needed, excellent support and are leaders in the bittorrent field.
Vuze is larger and uses a good amount of RAM. It has many more features available in the program and through its plugins. The latest being its "Devices" tab, which will convert (if necessary) and transfer to other devices such as iPod, XBox, Ps3 etc.. There is also the Vuze download platform , which offers legal downloads and an outlet for independent media producers.
uTorrent is the smallest bittorrent client (263KB) and uses the least amount of RAM of any bittorrent client. Yet it has all the basic features needed for torrenting. Their latest advance is the introduction of uTP transfers, which make throttling by ISPs difficult, if not impossible.
Both uTorrent and Vuze have excellent support through their FAQs, forums, guides (uTorrent) and Wiki (Vuze).
See the related link below, Best Free BitTorrent Client, for more info and links to downloads and set up guides.