I found that no matter what level i am the most XP I get is by doing all quests not just the ones that give the most XP. I too had this question so I did a test. Boss quests usually are the best because you get some from bosses and most of it from the actual quest Your Welcome
Doing main quests are extremely important in moving up levels. If you have already done a dungeon, doing it again will give you half of the experience of the first time. Other than that, battles give you about 30 experience depending on how long it takes and how many enemies you are fighting. The main way to level up, though every one has been saying it, is questing. Just keep swimming through the story and you'll be a higher level than you thought.