I don't know much about the movies sorry :( But Mew was utterly adorable in the first movie and the Lucario movie, and it proved to be an incredibly strong Pokemon indeed. <3 Mewtwo was kinda cool but a bit boring tbh, he was clever but most of what he did was expected; also I don't think he really was all that powerful haha. The Regi trio were great in the Lucario movie - they're my favourite legendary group so seeing them in action was awesome! They were really well thought-out with how they worked and their battle moves were very intense. Plus their speech was awesome haha. I don't really remember seeing and of the other movies... But I remember when Phione (if that counts as legendary, I don't even know anymore haha) was in the anime and that was incredibly cute <3 Though very stuck up D; And when Shaymin was in the anime it was amazing, plus super powerful - it was such a cute Pokemon until they went and changed its form! Although I guess it was somewhat dying so changing its form is okay I guess... I think Darkrai and Cresselia were in the anime shortly too which were pretty cool, though I barely remember Cresselia. Darkrai's battle in the league was amazing though, it faired so well against all of Ash's Pokemon, even Heracross with a huge type advantage, but eventually it got overpowered on the 5th Pokemon. Latios was also pretty cool in that same battle, but it didn't really get a whole lot of air time other than "Fly up! Now fly down! Shake Pikachu off!". I also loved the Kami trio in the most recent two episodes of the Rival Destinies anime; Landorus was so cool! I hated these three Pokemon before I saw them in the anime and now I adore them <3 Thunderous and Tornadus were such powerful (but stubborn!) Pokemon in the anime and it was great seeing them go all-out whilst attacking each other. And Landorus... Just wow; that thing was freaking impressive! Such a strong and resilient Pokemon really did deserve the role it had. <3 So yeah, that's all I really know about legends in the movies/anime!
Also what do people think of the Kami trio's new forms?? So excited AHH <333