I personally use tixati, primarily because whatever special sauce they cooked it with let's it run on the trashheap of a computer I use for torrenting when nothing else I've tried does. I'm not in any private trackers, nor do I really have a need to be, and like, not being usable on Mac is entirely irrelevant to everyone not using Mac. I personally don't care if it's proprietary, but that's just down to individual preference.

I'm not really sure what feature rich means since I don't have a comparison point, but there's a lot of menus with options in them, and I figure they all do stuff someone more dedicated than myself may care about lol.

Feature-rich

To be able to set download location, not download into folders, change location based on category, stop seeding after ratio or time, watch a folder for torrent files, delete said files after importing them, minimize to tray.

Not sure what transmission can and cannot do, but those are some examples of features in this context. Others may have a different opinion.