I am almost certain it is going to be Kamala Harris. She has the most star power, energy, etc.
She is pretty active in the Senate too. She is the primary sponsor on a lot of proposed legislation, including:
- Water for Tomorrow Act
- Saving Our Street Act
- Whistleblower Protection Act
- Votesafe Act of 2020
- Public Lands Act
- Housing is Infrastructure Act of 2019
- Preparing Superfund for Climate Change Act of 2019
- Accountability for Utility Executives Act
- Family Friendly Schools Act
- Domestic Workers Bill of Rights Act
- Accountability for Wall Street Executives Act
- Fair Chance at Housing Act of 2019
- 21st Century SKILLS Act
- Focus on Children Act
- Living Shorelines Act of 2019
- Outdoors for All Act
- 21st Century STEM for Girls and Underrepresented Minorities Act
And about 50 other pieces of proposed legislation in this and last congress.