In the beginning I used µTorrent, until it got fucked by whatever company that bought it. It was easily the fastest and most resource-smart client I've ever used. After that I tried a bunch, Deluge, Transmission, Azure and plenty more, but only qBittorrent seems to be a suitable replacement (and is open source as well). Now it has BitTorrent V2 support too :) You should really give it a try.