Cardcaptor Sakura will always be my favorite. Maybe it's because I saw the English dub when I was little and came back and watched it subbed nearly a decade later. I liked the animation style (yay CLAMP), the story was lighthearted and fun, and I absolutely loved the ending (the second movie ends the series; if you watch, don't miss it).
After that, I'd say Kanon (2006-07 airing) was very good, probably to the point of being #2. Absolutely beautiful story, I was simply bawling at a few points. The fan translation of the visual novel's almost done, too, so that's pretty exciting; I'll definitely import for that, the anime adaptation was just that good. I don't know why so many people dislike Key's works; I haven't disliked anything of theirs yet.
