As I previously recommended Ktorrent I feel I should report on that.
Lately I was not satisfied with Ktorrent anymore (although I would prefer it to Transmission any day) because it seemed still rather heavy and even speed seemed too low (or maybe other settings need adjusting ? - I only adjusted the upload speed and nothing else).
So, I tried qBittorrent and I recommend it, for the speed and also because it is lighter than Ktorrent. (Or at least gives that impression. Looking at the task manager, they are not very different, but I find qBittorrent more responsive and better adjusted - it looks like utorrent - while Ktorrent, while being a decent program has odd features like that you cannot close the program from the taskbar icon, it has tabs that seem useless and that you cannot close - well, in fact there's a close button to the right, it took me one month to see it, and so on.)
sudo add-apt-repository ppa:hydr0g3n/ppa
sudo apt-get update
sudo apt-get install qbittorrent
Then, I tried Flush, which is lighter, while still having a lot of features and looking basically the same. It surely deserves a try.
sudo add-apt-repository ppa:konishchevdmitry/flush
sudo apt-get update
sudo apt-get install flush
When having issues with a torrent client it is advisable to use multiple ones to test them.
Considering the clients with more features and settings, Deluge is one of the most used, reliable and with a lot of support (I mean wiki, faqs etc).
Another one that is very interesting is Tixati (here). It has A LOT of features for advanced users. (Don't worry, colors are customizable too :-))