The obvious answer is to go to the place that God has called you to go! It may be a place that you already have an interest in, a work or an institution that you or your church already supports. Pursue such avenues first.

Your choice may depend on a number of factors:

• Your cultural background and facility in other languages.
  If English is your mother tongue and you are not too good at learning languages, then East or Southern Africa, India or Pakistan may be a better choice than parts of West Africa (French) or Latin America (Spanish, Portuguese) – but you should always try and learn the local language in addition to the lingua franca.
• The particular medical experience and skills you have to offer.
  This is likely to determine whether you are more suited to a rural hospital or a teaching institution, a primary healthcare facility, or specific disease unit or project eg HIV, TB, palliative care etc
• The kind of organisation you want to work with.
  Be it church or mission hospital, academic unit, NGO or government agency