What level is the best?
get 650% GF+Luck, 200% Item Quantity and a hireling. Then farm in a map that you can1shot everything you kill preferably floor 500+.
Ty and @wat difficulty?
The highest difficulty you can kill stuff quickly in. To my understanding, there is no loot that is restricted to higher difficulties. Going up makes monsters harder to kill in exchange for higher gold, experience, and luck.
For example: Mythic 3 has +350% Luck and Gold Find. To get to the 650% @Clogon mentioned, you need 300% from gear. Dropping to Legend, you get +200% from difficulty so you need to provide 450% from gear to max out. But, the monsters on Legend are a lot easier than on Mythic 3, so you can trade some offensive stats to balance it out.
I see thanks
If you cannot oneshot the monster, then you need to lower the difficulty.