The tv series I watch dubbed, since English is what is most easily available to me, and I am accustomed to the cast. Though I don't mind watching an episode in the original Japanese with English subtitles if it's an important episode that was censored in the international release like The Legend of Miniryu/Dratini.
Now with the movies I frequently watch both subs and dubs. Since the movies are more story-driven I usually want to see it first as it was meant to be viewed in Japanese because often very significant plot content is removed from the US distribution, the first movie is a must viewing in Japanese as an example of this. Almost an entire half an hour is cut from the beginning, and that time spent with Mewtwo and Ai give the movie far more depth. There's more science-fiction and spiritualism in the original film, and the reason for this Mewtwo's creation is poignant.
Generally the Japanese voice actors are also better than the English ones in my opinion. In a typical episode of Pokemon where all of the Pokemon just repeat their names except for Meowth I don't notice this as much. However, since the movies always make rare, legendary or mythical Pokemon a central part of the story they are sometimes portrayed speaking like humans or communicating in our language telepathically. So if a Pokemon is going to deliver significant portions of dialogue then I am much more critical about what their voice sounds like. Sometimes a Pokemon sounds decent in either language like Entei in the third Pokemon movie. His voice in English great, it's a rich, baritone full of authority, yet warm when it needs to be in the interaction with Molly. I have watched and enjoy that movie dubbed. Other pokemon however are not so fortunate. The actor who voices Arceus in the English dub for 12th Pokemon movie is horrible. His voice is shrill and whiney, and all that screaming just makes Arceus sound unhinged. This dub should be avoided at all costs. Maybe there's another dub like Chinese or French where the Arceus is good--but this one ain't.
So the tv show 99% of the time I watch dubbed. The movies split 50-50 for me between sub or dub.