rtorrent is the best, especially if you care about your ratio. I run it on a linux box at home, within a detached screen session, so that I can manage all my torrents from my laptop at school, from upstairs, or wherever. Then, since this box is just sitting there all the time, it boosts up my ratio a lot more than I could if I used utorrent on my laptop, which is only turned on a few hours a day. Additionally, I can pick something out to download while I'm out, at when I get home, it will already have been going for hours.