Who is the largest independent broker-dealer?

Could you please clarify for me who is currently considered the largest independent broker-dealer in the financial industry? I'm particularly interested in understanding the size of their operations, their market share, and any notable achievements or milestones they've achieved that have contributed to their success as a leading independent broker-dealer. Additionally, I'm curious about their areas of specialization and how they differentiate themselves from other broker-dealers in the market.