Roulettes once a day then spamming your highest level dungeon available is best for tanks and healers.
PotD is better for DPS due to queue times.

I personally recommend doing a bit of everything to avoid burnout. PVP also gives exp, Clan Hunts and Beast Tribe quests in the expansions give decent exp, Leves and Fates aren't great but they still give exp and there are decent bonuses for them in the Challenge Log.