I love the first, best, because it's our introduction to the characters, and Jack seemed very well-fleshed out.
The second had more comic elements and, to me, was a joy-ride.
The third was (again, in my humble opinion) much more of a character-study. I appreciated, especially, the story arcs of Will and Izzy. They reached their natural conclusion in this one.
And, hey, I'm not one to complain about multiple Jacks.
All of that being said, I DO look forward to POTC 4. With reservations. As much as I love the character of Captain Jack, I think he and Barbossa only have one outing left in them that can be believable. And I can only hope that Ted Elliott and Terry Rossio (the writers) will be true to the characters and give us one last rousing adventure to go out on.
I think the dynamic of Captain Jack and Captain Barbossa is one that can be revisited one more time, and the story of their pursuit of the Fountain of Youth could be great fun.