Waymo is far ahead of any other autonomous vehicle company since they have far more operable vehicles. Also, their vehicles have driven a lot more kilometres--both in real life and in simulations. GM (General Motors) Cruise division has the second-largest amount of operable vehicles. Although their vehicles have driven fewer kilometres compared to Waymo, they've still driven a considerable amount.
Ford Motor Company's Argo AI, Tesla, and Baidu are also notable mentions.