Deluge
Until now I was using Vuze (Azureus), but seeing it become more and more bloated and with seemingly more bugs, I tried Deluge. I migrated all my torrents and it works fine, but I swallowed at some of the limited functionality in comparison with Vuze, mainly the queueing features. With Vuze, I could just enable all my torrents and it would automatically start the ones that were needed most for the swarm, according to its seed rank. Deluge determines its 'seed rank' solely based on seed time and ratio, as per the FAQ. Naturally, this is not optimal.
And Vuze has other functionality, such as automatically starting torrents with 0 peers, so that unpopular torrents would still register seeds at the tracker, for potential downloaders. And there are even more options such as exceptions and such.
I understand Deluge is much younger than Vuze, but is this something that is still going to be developed better in the near future? Otherwise Deluge is fantastic!