There's another thread by a similar title but it's 3 years old, so instead of necroing I figured we could share our findings from these 3 previous years.
So I found a village near my spawn (never seen it before due to savanna plateaus in the way), fortified it, built some farms and then started trading. Here I found some of the best trades I've ever got from villagers.
Best: Diamond pickaxe with Unbreaking III, Efficiency III, Fortune I, for a grand total of... 14 emeralds! I bought three, and I'll always carry one with me at all times.
Worst: An iron shovel for 7 emeralds. Plain, unenchanted, boring... for half the price of the pick mentioned above. Meh.
Honorable mention: Iron sword with Unbreaking III and Bane of Arthropods III for 9 emeralds, even better than the last tier trade (diamond sword with a measly Bane of Arthropod I, 15 emeralds).