The short answer probably is Iran, Russia and Hezbollah. A lot of overviews have been made on this topic but more attention probably needs to be given to the terminological question - what does "support" mean? Does it as in the case of China include preventing UNSC action? Should it include preventing support to Assads opponents? Balancing acts?
In the Guardian article below from 2015 an effort is made to be more nuanced. Further down - click on images to get to the source articles.