If you're only going to be travelling within London (i.e. not starting outside of London, and not visiting outside of London), then by far and away your best bet is either an Oyster Card, or a Contactless payment bank/credit card.
TFL have a very good website on the Oyster Card, with details of how to get one. They also have a section of the website for contactless which explains what kinds of contactless payment cards they do and don't support. Prices for Oyster and Contactless are, for most purposes, basically the same, and much lower than paper tickets. TFL have a dedicated Visitors to London section including a handy intro video.
One of the great things about an Oyster card or Contactless is that it'll start out in Pay As You Go mode, then if you happen to spend enough in one day that a travel card would have been cheaper, then it'll cap the cost at the travelcard fee for the rest of the day. It's valid on the tube, on buses, on trains within London, and the DLR.
If you're starting from outside London each day, then it may make sense to buy a travelcard as part of a train ticket to London. Depends if you're going to be doing lots of travel in London or not that day. However, you can't buy a ticket that's (Somewhere) -> London, returning the next day, with travelcards both days, you can only get a travelcard valid for a day return ticket. So, if you were to be doing a trip in, travel round London for a few days, then a return, you'd be better off with Oyster / Contactless.
With contactless, you need a bank card / credit card of the right type to be supported by TFL. Check their website to see if yours will be. Contactless has the advantage of offering a weekly cap too, which Oyster doesn't. Oyster has the advantage of being able to load weekly or longer travelcards onto it, can support discounts for railcards etc (normally only useful for UK visitors), and Oyster can be topped up in one go while Contactless charges once per day, which can make a difference for overseas cards with usage fees.
TL;DR Don't buy paper tickets for London only - use Oyster or Contactless, or possible a rail ticket with a travel card included in it.