I get tons of anime via BitTorrent and one of my favorite BitTorrent clients has been BitTornado (previously known as Shadow's Experimental client - nice rebranding too). I liked it because it was simple and light, and it uses the latest official BitTorrent code that allows for incremental allocation of files (since version 3.3). Before, BitTorrent would allocate the entire required disk space for the files you're downloading prior to actually connecting to peers/seeders.
Now, BitTornado 0.3 has just made an advocate of me. Changes include a per-file download priority system and most significantly, a fast resume feature that allows recently-run torrents to be started immediately without the need for a hash-check! I got my 4.4GB download (68% done) started in a flash, sans disk-thrashing. You can't imagine how useful that is with large torrents of several gigabytes, where hash-checking can take an interminably long time and not to mention the painful disk-thrashing involved. I get big torrents of several gigabytes quite often, and I dread the hash-checking process because it slows my system to a crawl for several minutes and also starts the processor fan spinning like mad to cool away the extra heat.
Also very cool (and something that brings it on par with other BitTorrent clients like Azureus in this respect) is the per-file download priority system which allows you to choose the priorities of files in a multiple-file torrent. You can choose to download just a single file in the torrent if you wish, something which was sorely missing before.
BitTornado's an excellent BitTorrent client, but to be honest I've only tried the official client, Azureus, and ABC, all of which I didn't quite grow to like. Which BitTorrent client do you use, and why?