Best overall flight search
Although not as full featured as others, Google Flights will get you the most quickly from "that looks like a 🔥 place to take ig pics" to actually seeing flight prices. How? By being extremely fast and easy to use, so typical Google. Intuitive price visualizations and price tracking alerts help further ensure you're getting a good price. At the very least, it's the best place to start your search.
Great for further price shopping
Among aggregators, Momondo consistently scours the most sources and finds the lowest prices, including for multi-city itineraries. Their Flight Insight tool is also a really nifty and unique way to understand how factors like seasonality, airport, day of the week, and others affect price. Our favorite for further comparing.
A good comparison of 25 flight searches showing how well Momondo does.
Plentiful filters and a useful price forecast
Have really specific flight needs? Kayak's abundant filters and flexible date inputs help you get really granular to find the right flight. Its price forecast feature based on its massive amounts of data is also really helpful on deciding when to book.
Best when flexible on date and location
Skyscanner generally finds low prices but where it really shines is when you're flexible on when and where you're going. For dates, it lets you search entire months vs. specific days. For destination, you can put 'Everywhere' and it'll rank places by price. Between these two it's impossible to not find the right flight to satisfy that travel bug 🐞.