Doing the Leveling Roulette once a day will generally get you at least a level, sometimes two. When that first-run-of-the-day bonus is active, it's the best XP available. After that, there's additional dungeon runs, sidequests, FATEs, and Palace of the Dead to get more XP when you need it.

As for sidequests, they give good XP when you're on-level for them, but it doesn't scale, so if you're much over the quest's level, its XP will be negligible. Most of the other XP sources (anything with level sync) will scale up the XP to be relevant for you.