I believe that a great science fair project is first one that sparks the creativity and a desire to learn about a particular subject or subjects in a child. Learning science can be a wonderful way to explore through multi-modalities. Having an opportunity to research a subject and then turn it in to a hands on creation can be an amazing and rewarding experience for not only the child, but the family, fellow students and all who visit the fair!
A trip to the public or school library is often a good place to start. Have the child talk about subjects that interest him or her. Find out what kind of learning they like best, such as subjects that are visually and/or auditorally engaging!
Support from parents and teachers also makes a science fair project go more smoothly. However, remember that the adults are assistants and should not drive the project themselves while the students look on passively. And a great science fair project need not break the wallet! Some of the best ones I have seen used common items, such as food dye.
I guess that in my mind a good science project will have conclusive results and leave no question at the researcher's mind about the objective of the project (although it might spark many other related questions and ideas for other experiements)