The best resonance structure of $CO_2$ contains a central carbon atom double bonded to each oxygen atom.
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This resonance structure is the best because the formal charge of all atoms is 0. This is the ideal formal charge. The other 2 resonance structures involve a central carbon atom single bonded to one oxygen atom and double bonded to the other oxygen atom. This is not ideal because the formal charge of the oxygen atom with the single bond is -1 and the formal charge of the oxygen atom with the triple bond is +1.