The four Gospels each tell the public life, death and resurrection of Jesus of Nazareth, and they are not four copies of one text: each chooses which episodes to include, in what order, and in what kind of language. They do not even start at the same point — Mark with the baptism of an adult, Matthew and Luke before the birth, John with a prologue and no infancy account. Catholic, Protestant and Orthodox Bibles agree on these four books and on their order: unlike the Old Testament, the list here is not disputed, and all three traditions open the New Testament with them.
Gathers the teaching into five long discourses — the first is the Sermon on the Mount — and ties the narrative to Old Testament quotations more than any of the others.
A good first read if you are coming from the Old Testament, or if you prefer the teaching gathered into blocks rather than scattered through the story.
MatthewThe shortest and the fastest: almost all action, one scene after another, with little sustained teaching — and its closing verses survive in more than one form, which most editions flag in a footnote.
A good first read if you want the whole story quickly: it fits into one or two sittings.
MarkThe longest of the four; it opens by stating its own method and its addressee, gives more room than the others to women, the poor and foreigners, and continues into Acts.
A good first read if you like narrative: it is the most orderly and detailed, and runs straight on into Acts if you want to keep going.
LukeA different order and a different vocabulary: seven “signs”, long discourses instead of parables, and episodes found nowhere else — Cana, Lazarus, the washing of the feet.
A good first read if you already know the outline of the story and want the contrast, or if you prefer a reflective text to a run of scenes.
John“Synoptic” comes from the Greek synopsis, “seeing together”: Matthew, Mark and Luke can be printed in three parallel columns and read across, because they follow the same broad sequence — Galilee first, the journey to Jerusalem at the end — and share a great deal of material, sometimes sentence for sentence. Sharing an outline is not agreeing on everything: the genealogies in Matthew and Luke, for instance, are not the same. John falls outside that comparison. It places the events in a different order (the temple episode comes near the beginning rather than the end), keeps a vocabulary of its own throughout, replaces short parables with long discourses, and gives much of the book over to episodes the other three never mention. How to account for the overlap among the first three is an open question among scholars; the overlap itself is something any reader can check.
There is no required order and no consensus. One reasonable route is Mark first — it is the shortest and lays out the frame of the story quickly — then Matthew or Luke to see that same frame filled in, and John last, where the contrast is easiest to notice. Plenty of editions and reading plans suggest the opposite, starting with John for its plainer vocabulary; and the canonical order — Matthew, Mark, Luke, John — works just as well. Any of the four is a complete entry point on its own.