The world best and fastest bittorrent downloader is Thunder. However, thanks God I am Chinese, I am an oversea Chinese. I am glad to become a Chinese because Thunder(迅雷) is a Chinese downloader which is amazingly faster than uTorrent, BitComet, Azureus 99999999999 times. It is amazing, you will not believe how fast is it unless you have tried it. Now we should know that why Chinese language is so important for us. There still have much more benefits that we can gain from China if we know Chinese.
for windows i think its pretty much utorrent all the way.
for linux, which i use, its deluge which is my favourite. although vuze is vigorously promoted, i dont find it appealing at all. its just way too much.