Dating an ancient document is never an exact science.  However, today general scholarship about the New Testament books holds that the short Gospel of Mark was the oldest Gospel.  The first letter of Paul to the Thessalonians was the oldest document, from around the year 50 CE.  Mark, with a hypothetical other source (Q=Quelle) that is now lost, became the source for the Gospels of Matthew and Luke.  The Gospel of John was generally considered to be the last of the gospels, around 90 -100 CE.