Dungeons, Daily Roulettes (levelling and 50 roulettes. Guildhest will offer a small amount of exp that becomes kind of useless in the later levels), and Main Story quests are what I found to be the most effective ways to level. Personally, I'm not a big fan of Palace of the Dead, but you might be, so go ahead and try it since it does grant EXP. As Frizze said, you'll be able to get into any dungeon you want almost instantly, which is always convenient lol.