If you have a bajillion gold then amulets would technically be the "easiest" due to having the largest item pool for one single type... Just spam on lvl 80+. Difficulty doesn't matter only character level matters.
same odds for all items
0.05% chance to be upgraded to unique (if there is a unique version of this item ofc)
it doesn't matter if there are 1 or 10 uniques for it...
so you basically just want to gamble at the lowest possible igg cost, but at a clvl that can still roll whatever unique you are looking for
lenymo, hotspur, hand of broc are probably the 3 cheapest ones to get from a low clvl