It's my fav too, because it has a lot of S+S in it, and the airport scene is so kawaii! It was so sweet, and the ending song with the memories was a really nice touch.
If I couldn't choose that one, I'd probably go for the episode where Sakura falls down an elevator, and Syaoran calls her name...it's such a pivotal moment in their relationship, and it's so sad to see Syaoran - it looks likehe's breaking down, unable to belive that she's gone....aaaah!