For my two kids, it's Pocoyo hands down. It's colorful, teaches a good lesson in most episodes, and the characters are beyond cute. Toy Castle is a close second (gotta love the ballet moves and the stories), and I believe Nihao Kai-Lan is third... even though the Mandarin could use some work.
Other honorable mentions would be 4 Squares, Dora/Diego, Miss Spider's Sunny Patch, Backyardigans, and This is Emily Yeung.
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"When you have a pretty smile, the world truly becomes much sweeter than the best candy."