Blue states have high-quality public defenders because they receive adequate support from the state. Certain Southern states also have high-quality public defenders, as a result of the civil rights movement (for example, Atlanta, Georgia).

In a lot of smaller counties (including in California), public defenders are private lawyers operating under contract (because there is not sufficient need for a full-time public defender). They're usually selected because they are extremely competent and/or have cozy ties to the D.A., but either way, they also produce good results.

Not all public defenders are good. In some states (specifically Ohio and most Southern states), public defenders are employees of the "county" but are paid minimum wage base salaries and receive the bulk of the compensation as commissions per-case (regardless of outcome). In those states, public defenders generally have little to no budget to hire experts or investigators, but face off against prosecutors who have essentially unlimited resources (and market-rate salaries with pensions). As a result, these public defenders are overworked and simply do not have the time or resources to do a good job on any of their cases.

If you're looking for the actual numbers, I don't know of a freely available source. There are usually a few law journals that publish these stats every year or two, but my current legal database subscription does not include access to these journals.