I have been using the official BitTorrent client, written in Python, for quite some time, but was never really happy with it. The user interface looks outdated, doesn't give much control, etc.
In the past I have used Azureus (written in Java), but was not very happy with the amount of memory the program was using. I also looked at several other clients, but somehow decided to stick with the official BitTorrent client, written by Bram Cohen in the Python programming language.
Until some time ago I discovered µTorrent (micro-torrent, or mu-torrent), which I updated today. It's extremely small, the download is just over 100 kbyte, (yes, kilobyte and hence does justice to the prefix micro) and it uses very little memory when downloading files using the BitTorrent peer-to-peer protocol and seeding. Yes, it sounds like I am talking about an application written 10 years ago, but µTorrent is new, updated frequently, and best of all comes with an automatic update system. Highly recommended.