Blingtron’s Secret Vault
Probably the best garrison mission due to its huge wallop of gold (though there is the raiding mission) at 500g. If you get this everyday you would make 15k a month, or 75% of a wow token at its low!
Usually the other gold based quests are also 3 follower but only rakes in 175 to 250 gold.
Note that if you have a treasure hunter follower it goes all the way to x2! (Which you can get randomly, through the inn, or Harrison Jones)
With that you would make enough for a wow token each month even at the base price of 30k.
And that’s not to forget the Blingtron daily that gives you a nice goody bag of gold as well as profession materials! Quite the bling bling (one easy way is to hop to Ashran and see if anyone has the robot out)