Stability of resonance structures
When considering a reaction in organic chemistry, there is a very high chance that an intermediate is formed and characterized by different resonance structures. However, it is important to remember that there is only one real structure per molecule, the resonance hybrid, while what we look at, are the different resonance contributors.
Often, these structures represent the movement of a charge around the molecule, which stabilizes the resonance hybrid. But, some resonance structures contribute to the overall stability more than others. For example, when negative charges are placed on electronegative atoms, like an OH group, the structure is more stable.
In general, resonance structures with the most atoms with a complete valence shell and the least number of formal charges are considered the more stable ones. When dealing with charges, we have the most stable structure when a negative charge is on the most electronegative atom, or a positive charge is on the least electronegative atom. In addition, the least distance is present between the charges, in terms of the number of bonds, the most stable the structure is.