What's the fastest/easiest way to get Forgotten Souls?

There are several ways for getting Forgotten Souls :

• Doing rifts : The rift guardian has a high chance of droping a Forgotten Soul. Pick the higher Torment level in which you can complete the rift quickly (2-3 min max). Farming in party will help you farm faster and in higher Torment resulting in more Forgotten souls. Moreover, the bloodshards you get from the Rift Guardian allow you to gamble at Kadala, significantly increasing your legendary "drop" rate.
• Killing an Odious Collector
• Salvaging unwanted legendaries (my trick to get "trash legendaries" is to really wish for something good, then the next 50 legendaries are going to be worthless). If you really need Forgotten Souls and have plenty of other crafting materials, you can use Kanai's Cube to upgrade rares into a legendaries and salvage them.

I usually clear T10 rifts and salvage every legendaries that is not usefull for my character. I currently have more than 8000 unused Forgotten Souls.

if ur on switch and have 2 switches for some reason you can create 2 profiles on both of them in non-seasonal mode, save your data to the cloud on ur main switch and download it on ur spare.then just drop all the legendaries for the 2nd profile, stash them and repeate until your 2nd profile has a full stash.save your 2nd profile to the cloud, download it on your main switch then have them play with your character on your main switch and drop all the legendaries. salvage all thosee lagendaries on your main account then repeat.