"With its growing Optum group of technology-enabled services, it's the best positioned for the long haul, too," Schafer writes. UnitedHealth's massive size and diversified portfolio may also be what led it to leave the largest industry trade group, America's Health Insurance Plans. The company also boasts its own pharmacy benefit management system thanks to its $12.8 billion purchase of Catamaran, a move that a combined Aetna and Humana may emulate once Aetna's contract with CVS Health expires, the two insurers' leaders said recently.