Having leveled many, many alts, I can tell you that the best way to feasibly level is a combination of questing and dungeon finder. Don't spam dungeons all day, every day, unless you just want to. Instead, most of the dungeons have quests with them that give a ton of exp. It's best to run each dungeon at least once to do those quests, as it maximizes how much exp you get for that small timeframe, and then spend the rest of your time questing. You will definitely get more exp doing normal questing, then from spamming dungeons. As such, anytime I'm leveling a new alt, I make sure that I tag every dungeon once, then focus more on questing (unless a dungeon has a specific upgrade I could use that would help). This method will level faster than either questing or dungeons alone.