The worst client has to be BitCommet because when I seed something, like I am doing right now with an anime, and only connected to one guy (who is downloading from me). He can have all my bandwidth, but because he is using BitCommet he doesn't ask for any of it. The bloody client is crap at seeding too. I mean I can give him this file now, but he doesn't want it.
I did try BitCommet once because it is the next most popular client to Azureus and the thing is a leach-O-matic. Basically it allows you to queue a load of torrents and then set a download cap for the number of torrents you have simultaneously downloading, and a seed cap where a torrent is seeded to a specified ratio then disconnected, but what I found was that after I had downloaded two torrents (I am downloading one at a time), it was trying to see all three (including the one I was downloading) so nobody was getting anything fast, and neither was I because I wasn't sending anything out!
Ye it downloaded those two torrents fast, but then I hit a gridlock situation and its just stupid. The thing actually encourages leaching by its design and admins have reported improved seeding if they manually block BitCommet clients in the initial seed.
So I am now using Azureus as always and it is still the best bittorrent client out there.