I've started to use BitLord. Pretty sweet. Used more for illegal stuff but it has some great features:
Intelligent Connection, Auto Optimization for different connections.
· Smart Rate Control, optimize the upload distribution to get the max download rate.
· Disk Allocating, no long-time disk allocation, decrease disk fragment to almost zero.
· Intelligent Hash Scan, no time-consuming scanning when seeding / resume.
Azureus is also a good one.