I only started watching DP until recently so I'm not caught up with it either. I started with the first season but have also seen random episodes of Battle Dimension and Galactic Battles. I've seen two of the movie; The Rise of Darkrai and Arceus and The Jewel of Life. I need to watch Giratina and the Sky Warrior to finish the trilogy of the DP movies. ( The Rise of Darkrai, Giratina and the Sky Warrior, and Arceus and The Jewel of Life are part of a trilogy that are related to another.) So far though, it's slowly becoming my favorite saga of the anime.
One episode I really liked so far is Like It or Lup It!. It focuses on Dawn trying to get Piplup to learn Contest Moves and I found the plot to be pretty well-written and somehow heartwarming to me.