No real answer
In my opinion there is no real answer to this question. Google flights gives you an immense flexibility and the possibility to make a lot search requests in a short time. So to get an orientation this probably a good start.
From there I usually head over to kayak to confirm the flights I found. And if I have the feeling that there is more saving potential you can try skypicker. They have a diffferent business model searching exclusively for hacker fares. However they are also integrated in the kayak universe.
But I hope to be able to present a completely new search soon that finds completely different fares.