Defnitely May's apperance in DP, no question about it. Hell, I'd even say that except for the announcement of Best Wishes! after that the Ash Vs. Nando battle had aired, the announcement of May making a return might just have been the thing that during the DP saga got me the most exicted. I was completely overjoyed.
Next I'd probably say Dawn's appearance in BW, because even though her cameo served no purpose I finally enjoyed Dawn's character, who I previously only felt hatred against due to her awfully boring personality. Her interactions with Iris were great, and the three first episodes with her she was actually really enjoyable, not to mention that she and Iris also had a great battle in the World Cup.
Misty's cameos I never really cared much for, so I'd put her in last place.
Next I'd probably say Dawn's appearance in BW, because even though her cameo served no purpose I finally enjoyed Dawn's character, who I previously only felt hatred against due to her awfully boring personality. Her interactions with Iris were great, and the three first episodes with her she was actually really enjoyable, not to mention that she and Iris also had a great battle in the World Cup.
Misty's cameos I never really cared much for, so I'd put her in last place.