First Movie: Released between episodes 18 and 19(Beggining of Don Krieg arc)

Second Movie: Released between episodes 56 and 57 (Apis arc. "Filler")

Third Movie: Released between episodes 102 and 103 (Arabasta arc)

Fourth Movie: Released between episodes 146 and 147(Jaya arc)

Fifth Movie: Released between episode 183 and 184(Skypeia arc)

Sixth Movie: Released between episodes 223 and 224 (Memory arc. "Filler")

First three movies are crap though. If you are ahead of Arabasta then you can watch movies 4, 5 and 6 without getting spoiled.