The third one is the best of the series, but yeah, they're short games, so just play all of them. Well, the first three. The fourth one isn't very good. Lucasarts adventure games were mostly amazingly good games, but Escape from Monkey Island... not even close. Compared to most of the rest of the games in the great Lucasarts adventure game lineup, EfMI looks pretty bad... merely average, in comparison to greatness. Oh well, at least the first three were great...
Oh, and if you (or anyone else) do really want to play EfMI, DEFINITELY play it after the other three. Seemingly half or more of the jokes in the game are in-jokes or references to stuff from the first three games. The first three games had occasional references, but nothing like what they did in the last game... each of the first three stand up well on their own as well as fitting within a continuing story.