I've used my fair share of clients. First Mainline (which was decent, but a bit basic back then), then BitTornado (a solid improvement, not a bad client really), then Azureus. Azureus was still Azureus when I was using it, and I liked it. Solid interface, lots of features, and an ability to work around the NAT problems I was having back then. I've tried it since, but don't like it now. It felt a bit bloated back then, now it's like having a blue whale on my desktop.
I dropped Azureus when I moved to Linux, and started using KTorrent. It was a nice client, though I missed some features from Azureus. It also seemed to have catastrophic memory leaks in the versions I used back then, eventually causing me to dump it for Deluge. I'm still using Deluge now, and it seems to have a little bit more than KTorrent in terms of features, and doesn't use nearly as much memory. The interface isn't exactly attractive, but I'll live with that.
So, long story short, Deluge is my client of choice.