Josh Shapiro is from Pennsylvania, a swing state in the US; as the governor of Pennsylvania, it has been hypothesised by some that he would have a home state advantage and give Harris the greatest electoral boost. He is a few percentage points ahead of the other plausible VP picks in this market, possibly because of this effect:

In political science, whether or not this home state advantage is real is somewhat disputed. Vox, Would picking Josh Shapiro actually help Harris in Pennsylvania?:

For years, most studies suggested that presidential candidates do not enjoy any advantage in their running mate’s home state. In 2016, the political scientists Boris Heersink and Brenton Peterson published a paper that challenged this consensus.

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Using large-scale survey data on individuals from all 50 states, Devine and Kopko are able to gauge the impact of running mate decisions on voters. They find that, after controlling for all other relevant variables (e.g., partisanship, race, age, gender, income, etc.), voters from the same state as a party’s vice presidential nominee are no more likely to support that party than voters from other states.

All options resolve N/A if Harris is not the Democratic presidential nominee.