Google Flights has in my opinion the best interface to initiate the search. But it won't show you necessarily the best price if you are flying international airlines. So I always end up in the airline's website locally connected via VPN. Sometimes the difference is absurd, tickets selling for $300 in the international version of the website go for $60 if you access the regional version or from other countries except the US.
I can confirm that this is absolutely true for LATAM Airlines. Booking a Brazilian flight (origin and destination within Brazil) is cheaper on LATAM's Brazilian portal than on LATAM's USA portal whenever I've compared them. You don't need a VPN: just look at latamairlines.com/br/pt vs latamairlines.com/us/en.
Ahh, but they are very sneaky. The Brazilian site only accepts payments with credit cards issued by Brazilian banks. They won't accept credit cards from the USA or other countries. You have to use the USA portal or international portal of LATAM if you want to pay with a non-Brazilian credit card, and you'll pay more for the exact same flight.
