The official client is pretty good. I like to use BitTorrent 4.0.2, because the later versions make tons of seemingly unnecessary network connections that make me a little wary (and BitTorrent the company seems to be slowly becoming evil...). The official client offers a neat mix of performance and features.
Transmission is probably the best out there in terms of performance, but it's lacking in features. Last time I checked, it didn't even show you how many seeds you were connected to with each torrent.
Azureus is another one I use often for its advanced features, like superseeding and the ability to download only certain files within a torrent. The downside is that it's a bit bloated.