The DBZ timeline is so full of plotholes that ignoring its universal logic is probably a good idea. From what I can tell, here's how most of the movies roughly fit in:
Dead Zone: Should be prior to the Saiyan Saga before Raditz arrived.
The World's Strongest: Post Saiyan Saga (due to Goku being able to use Spirit Bomb) but I'm unsure whether it was before/during the Freiza (Namek) Saga. They're seen on Earth so I assume it's before Namek, but then again, Goku's already healed which conflicts with the episode timeline. D:
The Tree of Might: Also, unsure whether this movie took place before the Freiza Saga seeing as how Goku can again use Spirit Bomb and still has his tail.
Lord Slug: Again, before the Freiza Saga. Still no hint to Goku turning Super Saiyan, plus Gohan still has his tail.
Cooler's Revenge: A more definite placed movie and should have fit in after the Freiza Saga, seeing as how Goku defeated Freiza and Cooler intends to reclaim his family's honour. Plus, Goku can turn Super Saiyan, so that's a given.
The Return of Cooler: Post Android Saga but before the Cell Saga. Goku and Vegeta can both turn Super Saiyan yet Gohan can't, which I assume should have occured during his training in the Hyperbolic Time Chamber in preparation for the Cell Games.
Super Android 13: Somewhere during the Android or Cell Saga, or else in between. Again, both Goku and Vegeta go golden and Future Trunks makes an appearance.
Broly: The Legendary Super Saiyan: Pretty much around the same time (before/after) Super Android 13.
Bojack Unbound: Should have taken place after the Cell Saga since Gohan is able to turn Super Saiyan 2 and because Goku is already dead. But from what I've seen, it's shown that Gohan reaches Super Saiyan 2 in the movie, which conflicts with the episode timeline. Again. Dx
Broly: Second Coming: Goten, Trunks and Videl's first movie appearance and in which both Saiyans are able to go Super. They're also searching for the dragonballs and occured wither before Goku taught them how to fuse or after Goku's one day return to Earth. Either way, this movie should have taken place during the Buu Saga.
Bio-Broly: Should be right after Second Coming. Episode-wise, it should have occured after the World's Martial Arts Tournament during the Buu Saga, as Android 18 can be seen with Hercule.
Fusion Reborn: Not really sure when this movie took place. Assuming Goku is in Heaven and Vegeta in HFIL, it should have squeezed in after Second Coming or Bio-Broly. :S
And that's all I've watched/know of the movies. I'm 100% sure I've got them wrong but with the myriad of canon plotholes already, it won't come as much of a surprise.
