From The Verge:
You'll know which ISPs are faring best because they're labeled as "HD Verified." This means that customers can expect reliable streams of at least 720p; apparently setting the bar at 1080p was too great of a challenge for these companies to meet. The methodology YouTube uses to grade ISPs is here.
Go here to compare.
Related: 5 U.S. Internet Service Providers whose connection ports are saturated refuse to upgrade their infrastructure.