I've watched them all so far.
The ones I liked were
One Piece movie 2: Clockwork Island Adventure
One Piece movie 4: Dead End Adventure
One Piece movie 5:Curse of the Sacred Sword (kinda meh but watch it yourself to decide)
One Piece movie 6: Baron Omatsuri and the Secret Island (The best one)
One Piece movie 7: The Giant Mechanical Soldier of Karakuri Castle (this one had good animation quality)
Also there were some t.v. specials but I haven't seen them all, they were hour long episodes basically.
This one is good. Adventure in the Ocean's Navel. Giant Enemy Crab!