Baron Omatsuri and the Secret Island was one of my favourites because it's main topic is nakamaship (friendship between the crew), and isn't that one of the reasons we all love One Piece? Well, at least for me XD
Strong World is awesome, I'm sure you've seen it by now after everyone told you to XD I recommend you to watch the special Strong World OVA before it, though, it's amazing and explains some stuff

Practically made by Oda himself, what can we expect except excellence? XD
Other movies that I recommend:
-Dead End Adventure
-Giant Mecha Soldier of Karakuri Castle
-Clockwork Island Adventure
-The Cursed Holy Sword (If you're a Zoro fan, you'll
love this one)
-One Piece :"The Movie" (first one)
In that order. All of them have hilarious moments (Nami trying to murder a child, Sanji pouring solar cream over Nami's body, Usopp faking his death, etc...)
Haven't seen the third movie (Chopper's animal island or smthing like that) so I can't talk about it
The 8 and 9 movies (Arabasta and Drum arcs covers, respectively) can get boring, it happened to many. Basically it's the same story, with one or two twists. In the 8, there a couple of extra scenes and the animation is better. It's basically watching the Arabasta arc a bit abridged in HD. If you like that, go watch it. As for the 9th movie, it's a "What if..." story, which would take place in an alternative universe after Enies Lobby (that is, with Robin, Franky and the Sunny), in which Chopper didn't join before Arabasta and Nami falls ill after EL, meaning it's Drum arc with Franky and Robin but no Vivi (and an extra boss, Wapol's brother (or cousin, can't remember)