The answer to this question really depends on what criteria you are using to measure and compare the different health insurance companies. However, based on current J.D. Power rankings and reviews, the number one health insurance company in the US is UnitedHealth Group.
UnitedHealthGroup is currently considered the industry leader in terms of customer efficiency, transparency of payment, cost, customer service and other relevant factors. In the most recent J.D. Power ranking for 2018, UnitedHealthGroup scored 863 points out of 1000, which was the highest score achieved. This score is based on an overall customer satisfaction survey, which covers different facets of health insurance, such as provider choice and satisfaction, communication, coverage and benefits, health plan offerings, claims processing and customer service.
For more information, you can view the full ranking on J.D. Power's website: https://www.jdpower.com/business/press-releases/jd-power-2018-u-s-health-insurance-plan-studies.