The fastest way to level up is to get power leveled by somebody who is already 70 and has good gear. If you're trying to level solo I think the fastest way is still to start adventure mode and look for any cursed chest quests. Do those quests, run one ore more high monster density areas (act 1 cathedral floor 3 IIRC) and restart. Then around level 30 switch to running rifts and when greater rifts open, run greater rifts when you have keys for them. GRifts are the fastest way to paragon level by far.
