Your daily Low Level Duty Roulette is the best source of EXP, once per day. Guildhest Roulette once per day also helps a bit.

Past that... doing your most recently-available dungeon, doing levequests, doing FATEs, are all sort of even-ish; and of course if you haven't exhausted them yet you've got quests too. If you mix up a variety of them all you can also score your Challenge Log bonuses, which are additional large chunks of EXP. Once you hit 41(?), the Beast Tribes start opening up with extra daily quests you can take.