For me Curse of The Black Pearl is clearly the best, it mixes a story about Pirates and the supernatural, and makes for all around entertainment. The story is a little silly but its not too silly and can be enjoyed by all the family. I would go as far as to say that its one of the greatest films of all time.
Dead Man's Chest is the weakest in the series to me, there were parts when it started to feel tiresome, for example the part on the island with the cannibals, for me that whole segment wasn't necessary to the plot, and it lasted about 30 minutes. It did have its good points though. I enjoyed the battle with the Kraken, and the ending is great, it really made me look forward to part 3.
At World's End is better then Dead Man's Chest, but it is very silly. There were parts when I was hanging my head and tutting, like the part in Davy Jones Locker where they turn the boat upside down, and also seeing all the multiple Jacks. Don't get me wrong I love fantasy movies but some parts were just ridiculous. But it did have some great parts, Barbossa and Jack's constant arguing about who is Captain of the Black Pearl was hilarious, it was nice to see them together again. Rush's presence was greatly missed in Dead Man's Chest so it was nice to have him back for this one. And the battle at the end was just EPIC.
I have mixed feeling about On Stranger Tides. A part of me misses Will and Elizabeth, they were both very important characters and there were times when I thought, I wish Will and Elizabeth would show up it doesn't feel the same without them. But at the same time I was happy that we could just focus on Captain Jack, because there were times in the second and third where he felt a bit pushed out by them. Also it was more like the first one, it was a simple plot without too many twists and turns, but it didn't have the all round feeling of EPICNESS like the first one and the third one had. But overall I enjoyed it, and the ending does a good job of setting up the fifth one. So here is my order of best to worst POTC films.
1. Curse of The Black Pearl
2. At World's End
3. On Stranger Tides
4. Dead Man's Chest