I think uTorrent is probably the most used client today. It's been my favorite ever since Azureus morphed into Vuze and became a bloated mess. One thing to beware, though, is that I spent about 2 months troubleshooting BSOD problems on a new computer build a year or so ago. It drove me nuts, and ultimately I traced it back to an incompatibility between the (then) new version 3 of uTorrent and my Realtek LAN driver. I downgraded to a previous 2.x version of uTorrent on that system and it's been running flawlessly ever since. I'm not even sure what version of uTorrent is out now -- maybe I would be okay upgrading now, but I've never felt the need.