I use ABC.

The only contenders in this game are, of course, ABC and Azureus. They're the only standalone BT clients that utilize a queue system. Azureus is awesome because you can use it to download specific files from the torrent, but it's nowhere near as lightweight as ABC. The new beta of Az even lets you set upload limits for individual torrents, a feature previously exclusive to ABC. BUT, the bottom line is it uses ing Java and it slows the crap out of my machine. Your mileage may vary, of course.

As for trackers...eh, well y'know.