In general, yes the GM is the most important person for any game. That doesn't mean that the players are not important, but individually they contribute less to the game. A crappy GM will likely find himself with few/no players, so they can't be a tyrant.
The GM wears 3 hats: author, storyteller, and judge. When designing the adventure, they are working as an author, trying to design the best plot and challenges they can. During the game, they start and end with storyteller, describing the setup and epilogue. During the game, they should allow the players to react to the premise/challenge, and determine the results fairly. Sometimes that last part gets lost in the desire to force a campaign to continue (or by a tyrant who lords it over his players).I always liked the term "referee" better -- it implies neutrality and doesn't have importance overly tied to it, as "Dungeon Master" and "Game Master" do. Yes, they run the scenario, but the best referees let the chips fall where they may without trying to influence proceedings.