Search engines are not made to find the cheapest flights. Cheapest flight only deals to be found to see that any particular company has in place at their Hot Deals section or in places such as supply SkyAuction.
I know this is not the kind of start you'd expect from an article in which I was supposed to promote search engine, but I just want to be sincere with you. This is not the best solution.
Once this is explained, for people who really have the time and want to find a quick alternative to buying their flight in a way that probably is not the cheapest but can find a deal that is more or less well in a very few seconds, I must say that it there is a search engine that puts together the other search engines plus all existing crucial airlines.
Qixo his name, not knowing of the most known search engines, but basically he would gather all the quests you can do with Cheaptickets, OneTravel, Travelocity and Priceline in a single operation.