List of GUI Based Torrent clients
Transmission is a good Torrent client, it is installed by default .
deluge 
What is Deluge?
Deluge is a full-featured BitTorrent client for Linux, OS X, Unix and Windows. It uses libtorrent in its backend and features multiple user-interfaces including GTK+, web and console. It has been designed using the client-server model with a daemon process that handles all the BitTorrent activity. The Deluge daemon is able to run on headless machines with the user-interfaces being able to connect remotely from any platform.
Deluge is not tied to any particular desktop environment and will work just fine in GNOME, KDE, XFCE and others.
The libtorrent library is a huge part of Deluge.
What about features?
Core/UI split allowing Deluge to run as a daemon
Connect remotely to the Deluge daemon
Web UI
Console UI
GTK+ UI
BitTorrent Protocol Encryption
Mainline DHT
Local Peer Discovery (aka LSD)
FAST protocol extension
µTorrent Peer Exchange
UPnP and NAT-PMP
Proxy support
Web seed
Private Torrents
Global and per-torrent speed limits
Configurable bandwidth scheduler
Password protection
RSS (via Plugin)
Other libtorrent features
Installation
Install via command line:
sudo apt-get install deluge
Install Deluge via PPA:
https://launchpad.net/~deluge-team/+archive/ppa 
ppa:deluge-team/ppa
Click here for instructions on using PPAs.
Comparing the speeds of clients I think this is a difficult question to answer and can only be really tested in lab conditions.
I am one of the Deluge devs and we get quite a few people that rave about the transfer speed however the credit should really go to libtorrent(rasterbar) which does all the heavy lifting and is at the core of Deluge plus many other clients such as qBitTorrent.
As for features there is a large table of torrent client comparisons on wikipedia
From a personal viewpoint I use Deluge torrent client as I perceive it to be the best client for Ubuntu, written in Python with an excellent server/client setup so can be accessed via web, console and gui. It also has a good range of plugins and is very stable.