I think most people would agree in saying that the first few movies were very forgettable, and it's the recent ones (which actually involve Oda-sensei on the creative team) which have been much higher quality offerings. So in some ways you might do best by watching the rest of the films in reverse release date order!
The 6th OP films is the first good one! I didnt know it was directed by Hosoda when I first saw it and was blown away by how much better it was than the ones that came before it. Though it made sense when I realised who had directed it. To be honest the 1st 3 films are more like OVAs given they are all under an hour long.