Short answer: UnitedHealth Group (UnitedHealthcare) is the most popular/largest health insurer in the United States in 2025 by national market share and enrollment — especially dominant in Medicare Advantage. (AMA-assn.org, KFF.org)

Details:

• National market leader: UnitedHealth Group is the largest U.S. insurer by market share (largest parent organization across commercial and overall lines). AMA data shows UnitedHealth at the top of national market-share rankings. (AMA-assn.org)
• Medicare Advantage: UnitedHealthcare is the single largest Medicare Advantage sponsor and, together with Humana, accounts for a very large share of MA enrollment (UnitedHealth and Humana together make up roughly half of MA enrollment). KFF’s 2024–2025 analysis highlights UnitedHealth’s leading role in MA markets. (KFF.org)
• Variation by market type and geography: “Most popular” depends on the market—Centene is the largest insurer on many public exchange (individual) markets, Blue Cross Blue Shield affiliates dominate many state commercial markets, and regional systems (Kaiser, local BCBS plans) lead in specific states or metros. The AMA and KFF reports show these regional and market-type differences. (AMA-assn.org)

Top nationally (examples from AMA/KFF source summaries): UnitedHealth Group (largest), Elevance Health/Anthem, CVS Health (Aetna), Cigna, Humana, Centene — with shares and rankings varying by commercial vs. MA vs. exchange markets. (AMA-assn.org)

If you want the single most popular insurer in a specific state, county, or market type (employer/commercial, Medicare Advantage, or ACA exchange plans), I can pull the local 2024–2025 numbers.