I am using less known client called BitSpirit. I tried Azureus, BitComet, ABC, BitTornado at times and in my opinion it beats them all. I've never heard of a feature in other clients BitSpirit doesn't have.

Official site:
http://www.bytelinker.com/intl/bs.htm

Although the latest version can be found here:
http://www.versiontracker.com/dyn/moreinfo/win/30786