Chronologically, Days is the second game because it starts a little before CoM does. In terms of release, Chain of Memories is as it was released on GameBoy Advance in between KHI and KHII. As far as main games, KHII is the second game.
The first game is easiest but then on you have the nobodies, the time travel concept, the duel wielding and identity concepts to explain. And the difference in Unversed, Heartless, and Dream Eaters. As well as the two colors of the moon of Kingdom Hearts.