As discussed in several other threads, the consensus seems to be this:
1) If you only care about loot, speed farm the level where you can one-shot mobs (Normal/Hard for many people)
2) If you want XP, try to play at a level where you can farm at a reasonable speed. At Torment 1 you will level very fast, if you have the gear for normal farming (T1 typically requires about 100k DPS)
The thing about the first point is that there is a slight increase in drop quality in Torment, which increases with increased T-level. But the mob HP will offset this with too much for it to be efficient.