The first one is really good and has a nicely paced, well written self contained story.
The next two movies in the serie are obvious cash-grab movies after they realized the first one actually was massive success and they had to slap something on it. It basically becomes a serie of Jack Sparrow memes that then tries to pretend there was always this 'trilogy' of movies worked out around to support his antics (Whereas in the first one he was a good character amongst the others). There are some good scenes in the latter two movies but all in all it's clear recyclement of what the studio suits think made first one popular.
I wish I could comment on the fourth movie but I've never managed to watch it to end without falling asleep somewhere midway. Might as well call it "Milk Jack Sparrow Dry" the Movie.