Which area you farm is decided by the area's monster level, not by the item —
the rule is the same for everything, so it lives on the
mechanics page.
NAMED TARGETS
Named monsters that can be farmed deliberately. The list spans all three difficulties;
each monster appears once, at its most rewarding one.
The figure is an estimated average kill count.
Duriel
normal · Lv 22~1 every 523 kills
Mephisto
normal · Lv 26~1 every 868 kills
Diablo
normal · Lv 110~1 every 918 kills
Diablo
normal · Lv 40~1 every 918 kills
Baal
normal · Lv 60~1 every 979 kills
Bloodwitch the Wild
normal · Lv 14~1 every 1,500 kills