The easiest to use app and it does everything for you.
Google Flights
The fastest and easiest to search a lot of options.
Hipmunk
One of the cleanest airfare search tools.
Hopper
One of the most sophisticated airfare search tools.
Momondo
The best flight analytics - a graph that shows price trends.
Skiplagged
If time is not an issue, this airfare search tool could find you a bargain.
Skyscanner
One of the largest and simple interface.