Joe Biden should select Rep. Val Demings of Florida as his running mate. She acquitted herself very well during the House Impeachment proceedings. She was an impressive questioner. I can imagine her being an impressive speaker at rallies. (See video below)
She is a solid Democrat with the right sort of ratings and endorsements. (See link below)
She was a police officer and a former Orlando police chief which I suspect would appeal to suburban voters of both parties. As an African-American and Floridian she could help Biden to win Florida and neighboring Georgia which are leaning Republican according to Larry Sabato's Crystal Ball. (See link below)
As a potential double first (first female VP, first black VP) she would inject added excitement into the campaign.
Hillary Clinton lost Wisconsin, Michigan, Florida, Pennsylvania and Ohio partly because the African-America voters didn't show up at the polls as they did for Obama. Val Demings could correct that problem.