Rifts to get GR keys.
GRs to get legendary gems / level them up.
Bounties to get materials for reforging in the Cube.
Aim to be doing your rifts in 2-4 minutes per run, your GRs likewise (at the highest level you can complete them in that time), to maximise the number of legendaries you get for time spent. As ancients are 1 in 10 legendaries, and primals are 1 in 400 legendaries, the more legendaries you get, the more chances for ancients / primals you have.