uTorrent 2.2.1 (Build 25302) was the best upon release and it still pretty much is the best. Other clients are equal to, but none are really better than this old horse.
qBittorrent is great also, it's the best client if you are on anything other than Windows (OSX Linux etc) and is basically just uTorrent 2.2.1 remade after uTorrent went all stupid. If for some reason you don't want to use uTorrent 2.2.1, and like most people who use qBittorrent, think this is a godsend after dealing with uTorrent post 3.0 when in reality this is just uTorrent 2.2.1 repackaged.
Deluge is also pretty sweet. However unlike qBittorrent there are actually reasons to use Deluge over uTorrent 2.2.1 if you are on Windows. It has awesome plugins that can do extra stuff that is mostly unneeded, and sacrifices some of the great things that q and u have, for those other great things. To each their own but its worth a try.
Transmission is OK, but no reason to use it on OSX instead of qBittorrent really. And the Windows version is the same, no reason to use it over the 3 above.
Vuze sucks
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