I want to know which are the most stable furan resonance structures. I know that structure number 1 is the most stable, but which one comes after?
A quick post so you can get started on thinking about this.
Remember, resonance is just a model. The electrons are delocalized all the time; the furan does not morph back and forth among the various structures.
You can do a "formal" evaluation of which resonance forms have the most contribution to the structure by looking at formal charges, and there are rules to help you decide, from the formal charges, which resonance forms are most important.
If you're looking at reactivity, the property you're interested in may not be that the one that is "more stable is less reactive". What you might look at instead, is the consequences of attack at the 2 and 3 position, look at the stability of the intermediates formed...a more stable intermediate would tend to drive the reaction along that path.
After looking at these ideas, feel free to come back with specific questions.