There's no question about it: the best way to find a cheap flight deal is on the Internet. Already a few years ago, a travel agent told me "Nowadays, it is just impossible for a travel agent to compete with the Internet to find a cheap flight."
Unfortunately, there still isn't a super Internet site which, in only one step and a few seconds, can provide the cheapest flight among all airline companies and for any destination. There are just too many variables. The majority of search engines for plane tickets have their own specializations. A specific site, for example, can be excellent for domestic flights in the USA but not for international flights.
Depending on your departing city (e.g., London or Bangkok) or country (e.g., India or Australia), it is a different site that is best. Moreover, your destination also has an influence on which site is most suitable to search for your flight. For example, if you travel to Asia or Australia, you will have to use different search engines than if you travel to Africa or America. To complicate things further, there also exist low cost carriers such as Ryanair, Easy Jet, Jetblue or Jetstar, which are usually not covered by the main search engine sites. Low cost carriers are airline companies that offer fewer services and don't serve all destinations, but they offer much cheaper flights than normal carriers.
So, the question is: what is the best site to find the cheapest airplane tickets? Unfortunately, for all the reasons above listed, it is difficult to answer. But I would say that http://www.kayak.com is the best in perhaps 50 percent of the cases. It covers over 140 travel search engines and can often provide tickets at low and competitive prices and this, no matter your departing point or destination in the world.
It is an excellent site to start your search with; you can even make a search using flexible dates. Another site that is worth using in parallel with Kayak and that is comparable to it is http://www.mobissimo.com.
But remember that even though Kayak and Mobissimo are often a good source of cheap flights, they will not always give you the best price for your ticket. For the other options, it takes a little time and experience to get to know where to search for the best price. Different books and resources are available on this topic. They are worth looking at and could help you save a lot of money on your next trips.