When travelling into London, what mode of transport do you prefer to take? Does it depend on the purpose of your visit (work, leisure), the price, the convenience? Did the congestion charge stop you driving into the city?
If you are in London for just a day, a great way to travel is by boat along the river Thames - the views are great and its fairly inexpensive. For more info see: http://www.tfl.gov.uk/modes/river/
Otherwise the tube is the best way to travel around London. I would advise purchasing a day pass (approx £8 for zones 1 and 2) or if you are staying in London for more than 4 days it may work out cheaper to purchase a weekly pass for approximately (£32). Check current prices on: http://www.tfl.gov.uk/fares-and-payments/fares