This probably isn't the answer you're looking for, but the best question for a science experiment is, "Is my hypothesis correct?" or, more fundamentally, "Is my idea correct?"
 
The basis of scientific experimentation is to construct a theory, a hypothesis, an idea, a conjecture and then put it to the test with an experiment.  Unfortunately, we're not taught much about where these theories and hypotheses come from - it's almost like a magical step in the scientific method.  But, once you have one, you can usually create an experiment to test it.
 
* Oil in the wheels of a toy car will make it roll down a hill faster than water in the wheels.
* Hot water freezes faster than cold water.
* Bread rises less as the amount of salt in the dough is increased.
* A fan with oval blades will spin faster than one with round blades of the same surface area.
 
These are hypotheses that can all be tested with an experiment.
 
Hence, the best question is, "Is my hypothesis correct?"