For me, it's Gotta Catch Ya Later.
You've gotta admit that although this is very very sad, it still feels good to watch this episode. Unlike the newer episodes, "Gotta Catch Ya Later" gives us a tearful goodbye to Misty. May and Max's goodbye wasn't very heartfelt and in the Black and White ending it didn't seem as if a Ash really missed Iris and Cilan.
Serena's one definitely was kind of sad, but that didn't matter much, to me at least.
In this episode, they give us a great ending.
1. We get a sweet and gentle song from Misty
2. We see Ash actually showing his emotions for Misty
3. And then in the end, you come to find out the real reason Misty stayed with Ash.
These are the reasons why I liked this episode. Besides Dawn and Brock's goodbye, the newer goodbyes didn't feel the same. Especially since the newer characters didn't have a real relationship with Ash and basically felt like just plain old replacements for the departing characters, Serena being an exception, I guess..
It was also the first time television irked some real emotions in me. Sadness, that is. Wasn't able to smile for the next few days.:(