I can't really decide my favorite, but a really good one I've read is called the Wings of Fire series. The story is pretty much "In the slightly magical land of Pyrrhia, dragons are the dominant species, as scavengers AKA humans hide in the shadows. Dragons are in a period similar to the Middle Ages, and there are seven "tribes" that rule the land. Mudwings, Sandwings, Rainwings, Seawings, Nightwings, Icewings, and Skywings. One night, Oasis, Queen of the Sandwings, hears rustling in her castle and goes to inspect, only to find her treasure room empty! She goes outside to see three humans that miraculously kill Oasis and steals all her gold and jewels. Afterwards, her three daughters find her dead mother and immediately launch into a feud for who gets the throne. The whole thing is blown out of proportions, and soon all the tribes (except the Nightwings) are allied on one of the three sister's sides and war breaks out. One Nightwing, Morrowseer gets a prophecy that pretty much says, "in twenty years, five kids who were born on the brightest night (when all three moons are full) are going to stop the war. Two of the Sandwing Sisters die, while the third gets to be queen." Now, Tsunami, Glory, Clay, Starflight, and Sunny must escape the cave which they have hidden and lived in their entire lives, meet their families, save the world, and discover the bloody and horrifying secrets that this world has to offer." I seriously recommend this series, especially if you like the Warriors series. It's in the same group of "probably for kids, but it's so gory that it's debatable." Right now there are six books out.