I'm assuming we aren't counting the Alternamorphs books? Because they would definitely be the worst! They read like a bad fanfic!
The best by far was The Andalite Chronicles. In fact, I'd say that's one of my all time favourite books period (not just in the series). Elfangor was a fantastic narrator, the story was great and it answered a lot of questions. It had action, humour, drama, it had everything!
The worst? I'd be tempted to say The Ellimist Chronicles. There were parts of it that I loved, and it at least explained a few things about the Ellimist, but other parts were a little confusing and it still left a lot of unanswered questions.