Torrent client for Linux? KTorrent! KTorrent is based on the interface that the most popular client for Windows - uTorrent provides, but it is not just a clone, KTorrent cannibalizes uTorrent and improves upon it.
KTorrent is built using the KDE library, and hence it is a KDE application. Of course like thousands of other KDE apps it runs on Gnome. However, typically when people (including me) are using Gnome they prefer to use Gnome applications. Here's why I've chosen KTorrent over other linux torrent clients, all of which you can download through apt-get or synaptic.
Bittorrent/Bittornado - Too simple a client. Doesn't have most options, the Gui is very limited. Only reason to use it would be for really old computers which can't power newer GUI, but there too you're probably better off with rTorrent.
Get it in Ubuntu: sudo apt-get install bittornado-gui
rTorrent - Text based torrent client. It can run the main torrent daemon at the background, text (ncurses) interface connecting to it as and when necessary. Very powerful, but I found the text based + scripted interface too complicated. If I had no other option, I'd probably spend more time on learning to use this interface and go with it. Only one caveat: it doesn't work with NTFS drives.
Get it in Ubuntu: sudo apt-get install rtorrent
Deluge - This is one good client for Gnome. However, since it is built using Python which can be slow in slower computers, and it follows the Gnome 'keep it simple' philosophy, the developers are not ready to add more GUI features. I've myself submitted a patch which would show a torrent bar like uTorrent/KTorrent where you can see which exact part of the torrent has been downloaded as opposed to a single statusbar. I and some other developers thought it was cool but one of the project owners was reluctant to accept it - so I concluded it's not the client for me.
Get it in Ubuntu: sudo apt-get install deluge-torrent
Transmission - This is the default torrent client for Ubuntu Hardy Heron to be released on 2008 April. I seriously do not know why this was chosen, as transmission doesn't yet support DHT. For those who don't know, DHT means distributed hash table, a method which allows a torrent to go on when once started without requiring the main server. If a torrent client doesn't support DHT, it won't be able to search for peers to connect who are outside the list of servers it has, in effect reducing the speed. Irrespective of your Internet connection bandwidth, most often you'll see a dramatic increase in speed for a DHT supported client than a non DHT. So Transmission is a strict no no. First thing you do after installing Hardy is remove Transmission and install something that is more capable, like KTorrent.
Get it in Ubuntu: sudo apt-get install transmission-gtk
Azureus - Never used it myself, but the complaints I've got about it is that it's a major resource hog. Also since it's written in Java, you'll the Java VM loaded in your memory as long as it runs.
Get it in Ubuntu: sudo apt-get install azureus
KTorrent - Now this is the one torrent client for linux that has all. Doesn't matter if it is made for KDE, the package manager will install required KDE libraries automatically to let it run in Gnome. Now you have access to DHT, NAT traversal, support for loading large ban lists, a very nice and informative GUI, hoard of configuration options that you can set without needing to script, and the ability to control KTorrent from terminal. The last thing is what I needed the most, when I figured out how I can connect to my computer from a remote place - as I wanted to control the torrent client remotely. This is how you control KTorrent from terminal:
Once you have a copy of KTorrent running, you can use the "ktshell" program (installed with KTorrent) from the command line, to control it from terminal. If you're connecting through SSH, you'll have to type the following line to let ktshell know which display KTorrent is running: "DISPLAY=0; export DISPLAY". And then you're all set!
Get it in Ubuntu: sudo apt-get install ktorrent
Verdict: KTorrent. If your system is really old and can't run KTorrent, and you're not writing into NTFS disk, then go for rTorrent.