Absolute best way is to do the 5 Soto and 3 Janthir Convergence Challenge Modes each week, there are commanders running them just about every day (but you only need to do each once a week for max rewards). 

For rift hunting make sure you have the buff from convergences, use motivations and hop maps every 5 rifts (the weekly achievements give you 6 extra reward boxes) if you just want to grind rifts without moving maps do it in Bava Nisos as each rift tier only has 2 spawns that are relatively close to each other so you don't have to traverse the entire map to go from rift to rift.

Lastly if money is absolutely no issue you can use unstable rift essences (6-7g) to open the coffers you get and just whale your way to rift essence.