I guess it really depends on child's age and level of maturity as well as interests. My seven year old likes "Magic Tree House" series because she is learning about different historical places and famous people. She also loves old fashioned classic fairy tales, especially ones that come from Eastern Europe.
My younger 4 year old likes books with animals as characters. Her favorite is "Franklin the Turtle" series. She also likes books about ballet and ballerinas and fairy tales. Both of them like books about elves, fairies, dragons and brave heroines. Especially when a female main character is perfectly capable to save herself and others, not a damsel in distress.