DMs don't have levels...
Yes they do. DMs earn xp by killing off player characters (tripple points in the PC suicides as a result of a storyline event), 30xp per Player you successfully rail road without them noticing, and 150px per Player that ends up doing something that is benifitual to the BBEG's plot.
FYI, the quickest way to level as a GM is not by running Call of Cthulhu, Ravenloft, or certain tournament styled adventures since they were designed to have a high mortality rate. However, if you would like to know, the game that earns DM's the highest amount of EXP was the My Little Pony RPG that was created at WoTC back in 2006, and later buried in a hidden vault under Area 51 before WoTC disinformation officers stated it was a "April Fool's Joke."
See, according to a nameless former employee at WoTC, the game awarded GM's so much EXP that one intern nearly achieved godhood after doing a TPK of WoTC's RPG design staff uriong a marathon game played on Saint Patric's Day weekend. According to my source, the Intern started summoning demonic horses and had to be put down using a shotgun loaded with glass chits. Afterwords everyone involved in the game vowed never to let knowledge of the event be known by the general public*...
* Unfortunately, as a result of some firings and layoffs, knowledge of the event was released.