I have essentialy used two decks since the last few years -The Rider Waite and the Mythic Tarot. I have recently acquired the Thoth deck.
Rider Waite is the most standard and popular deck. It is an essential for all beginners and even advanced readers. Directly or indirectly, it forms a basis for many other decks. The Rider Waite was my first deck about 6 years ago and I still use and learn from it.
The cards were created by Pamela Colman Smith under the instructions of A. E Waite.
The Mythic Tarot is based on Greek philosophy. I enjoyed reading stories about Greek and Roman gods as a child and I think this pulled me towards the deck. I have been using the deck for about 4 years now and am very happy with it.
The deck has been created by Juliet Sharman-Burke and Liz Greene.
Thoth Tarot by Aleister Crowley and Lady Frieda. It however went out of print for quite some time. I have recently acquired it and will begin using it in readings soon once I have established a comfort level with the cards.
To know more about tarot see http://en.wikipedia.org/wiki/Tarot