Which airfare site is easiest to use?
Google Flights. It is the fastest to search, the clearest to read, and it does not run countdown timers at you.
What makes one easier than another
An agency site earns money when you book, so its interface is built to move you toward booking: urgency banners, seats-remaining counters, prices that exclude the bag fee until checkout. A metasearch site earns money on the handoff, so it has less reason to hurry you.
Google Flights is the least cluttered of them. The calendar and the price graph are one click from the search box, and the fare it quotes is the fare, with the baggage rules shown before you commit.
When to use something else
Skyscanner and Momondo will search a whole country or a whole month at once, which Google does less well. Skyscanner's everywhere search is the right tool when you know the dates and not the destination.
Kayak sits in the middle: more filters than Google, more clutter too, and it includes agencies Google leaves out.
Common questions
What is the easiest site to book flights on?
Google Flights to search, then the airline's own site to book. That combination has the fewest steps and the fewest surprises at checkout.
Which flight site is best for finding somewhere to go?
Skyscanner. Its everywhere search takes a departure airport and a date range and returns destinations sorted by price.