I say I.
I is a letter that is used a lot, especially in the middle of words where it has several different sounds. Say: it, Idaho, marine, and bird. As you read books with children, point out how many times the "I" and "i" appear, and soon they'll be looking for them, too. I is mine.
What better letter to personalize than I? Children could cut out a huge outline of I and fill it with drawings of their favorite items (toys, foods, etc.).