I am doing a science fair project on plant growth. The question I have to answer is "What material grows the best plants?"

That question isn't easily answered. Where are you located? What plants will you be growing? How much light will you have to grow the plants. I'm sure there are a lot of other questions, but we can start here. Are you thinking of trying different fertilizers?

That's a very broad question. You'll need to narrow it down a bit, as what is best for one plant can be fatal for another.

Perhaps focus on certain food crops or a favourite type of flower and then do some research on what soil or growing media they prefer.

I agree, we need more information. While we wait, here is a link to a similar type of project that might give you some ideas.
http://learningandyearning.com...

And as this is a Science Fair project for you to research and experiment to answer the question "What material grows the best plants?", we can only make some suggestions and offer guide lines, not give you answers.

I would suggest you narrow down your queston and choose a plant, i.e. "what is the best soil for tomato production?". Or "what fertilizer works best for cucumbers?". Remember to keep all other variables the same.

So:
1. Ask a question
2. Research the question
3. Formulate a hypothesis
4. Test your hypothesis
5. Analyze your data
6. Draw your conclusion
7. Win First Place in Science Fair