As a place to spend emeralds, Librarians' book trades make them a commmon favorite. Clerics are a likely second (Redstone, Lapis, Glowstone, Enderpearls).
As a profession from which to get emeralds, Farmers are probably first (with Butchers second) as the greater number of items they will purchase makes it less likely one will be forced to buy unwanted items to reset the trades.
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WARNING: I have an extemely "grindy" playstyle; YMMV — if this doesn't seem fun to you, mine what you can from it & bin the rest.
Farmers are definitely the best way to get emeralds. Librarians are great for enchanted books like Mending. I didn't think they offered Mending for less than 10 emeralds. I also really like getting enchanted diamond gear from Blacksmiths. Here's a handy chart you may have seen that shows every trade each villager offers and the best possible trades.