I only read the first one. Still, False Prince has been far and away the most popular MG (YA-ish?) fantasy I've seen in a long time, at least with without an MC who's name ends in 'otter' or 'ackson' (or whatever those Land of Stories kids names were).
I'm kind of obsessed with Harriet Hamster Princess right now. Ursula Vernon is amazing, and I loved Castle Hangnail (and it's got some good messages about friendship and bullying you don't hear enough). But Harriet is better. She's kind of a jerkass, and always too school with everything she does. Plus, she's dangerously genre saavy, but so is everyone else in her fantasy universe, so it doesn't always work out for her. Did I mention she rides a Battle Quail, who is also a knight in his own right?