What is the lowest-cost way to transfer money between countries?

There is no set answer to this question, each corridor (the term used to indicate the transfer corridor between two countries) can be different from the next. Society for Worldwide Interbank Financial Telecommunication (SWIFT) is perhaps the most widely adopted money-transfer scheme used by banks, which charges pretty much a flat-free to move your money from a bank to bank. However, when dealing with multiple currencies, the banks that move money, may set your rate.

Individually, options for money transfers between countries vary. For example, from US to India, Xoom (company) is the preferred method, giving you perhaps the best rate for your US Dollars. In other corridors you will find very niche players operating or many a times, none at all.

Peer to Peer transfers are taking ahold of the the market, but their coverage is limited. You might want to check with companies like Currency Fair or Transferwise and see if they are available in your area.

It is always a good idea to ask around and research. Some preferred the sure and sound method of bank to bank transfers using SWIFT, whilst others may happily be able to use Bitcoins or Ripple for transfers.