Tixati's my favorite, but mostly because of all the pretty graphs. I think the idea is that it's supposed to be the "expert" torrent client that will show you every detail about everything, but the bittorrent protocol is simple enough that having all the extra details doesn't really let you do anything special.
But it does let you do things like automatically categorize torrents by primary tracker and give them different settings, or automatically filter out or prioritize files by pattern. General useful stuff.
Not open-source, though, if it matters to you. It was also banned from a bunch of private trackers for some inscrutable reason once 10 years ago, but I don't think that's a problem anymore.
EDIT: I'm not sure why I thought it was Windows only. Looks like it was always Windows and Linux.