Best website for international flights?

I'm wondering what websites you guys use and where you find the best deals to travel to and from Japan. I've used a few sites before, but just wondering if there are any that are the absolute best to use for good prices.

I wouldn't rely on just one. A lot depends also on when you're flying and from where. Sometimes you can get good deals buying straight from the airline. I've flown via Seoul many times and got a very good deal with very little layover. Others offer some dirt cheap airfares but require a grueling long trip to China with 12+ hours between flights - no thanks.
There is no bumper sticker solution for good deals.

Agreed. I look at expedia to get ideas of flight duration, arrival times etc and then go to the airline direct. For example for our latest trip, (pre coronavirus) Qantas direct leaving at about 10AM and arriving at 7PM or so (meaning you dont get into town until around 9PM) was going to be nearly $4500 per person business class. China Air (the Taiwanese one) has a better business class but leaves at 11PM, a 2 hour stopover in Taiwan then a 2 hour flight to Kansai arriving at about lunchtime, and it was about $3300 per person. While it is a nuisance to have the stopover, the better cabin and better arrival time, flying direct into Kansai, and cheaper tickets made it a no-brainer to go China Air. Of course since then the coronavirus has put some doubt on the wisdom of that...

For long distance and destinations that require transit, I often use Expedia or Tripadvisor, because their layout makes it easy to compare things. Of course, at the end, I try to book directly to the airline, because they typically offer the best deal, in terms of price and cancellation policies. Same goes with hotels.

But I usually start my search with only one or two airlines in mind, because service and comfort differs greatly between airlines nowadays. And when you're stuck in a plain for hours, comfort sure counts. I ask friends who have actually traveled the route to see which airline suits me best for that destination, and study each airline's official website for more details.

For shorter distances, I mostly travel free using my miles, so my millage website is all I look at. I notice that many frequent travelers keep choosing cheap airlines, but if you travel frequently, I just can't see why you don't rely on miles. And there are so many ways to collect your miles other than by flying.

Google flights is pretty good.