My Favorite: Scratch
Why? It allows students to be creative and solve complex problems. It also has amazing extensions that can add complexity.
Grades 3-5: Scratch Creative Computing Curriculum
Other Awesome Coding Options!
Code.org - Probably the simplest place to start, with great "unplugged" activities, easy lessons that gently step students from one concept to another, and good navigation. Can start as young as kindergarten with this and advance through middle school.
Bricklayer - Grades 3 and up; combine math, coding, and art
Pencil Code - Grades 5 and up; create art, music, games, and stories using block coding