As mentioned over on the page all about Emeralds, the Place of Legend with a 100% chance of bringing back emeralds every time (and with the shortest adventure time) is the Fairy Falls, and only when you use Clover. However, not everyone has Clover, and sometimes even when you do, he's out exploring other places. So I wanted to elaborate on the next easiest way to get Emeralds. Whether you use these instead of Clover/Fairy Falls, or in addition to, is up to you!
Note: Clover will also bring you back an Emerald every time he visits the Flower Tower, Dragon's Clutch, Pirate Wreck or the Well of Wishes, so if he's already out exploring one of those places, you can count on getting one from there.
The Dragon Ruin is the next best place, with its 4 hour exploration time. You can use a combo of Buttercup and one of: Bluebelle, Rafael, Bayoo, Spirit or Iris (seasonal dragon hero - not everyone will have) for a 100% chance of bringing back the green goods. If Buttercup is otherwise engaged, here's a list of the percent chances of all the best characters, so you can pick the best two for you:
Everyone else has a mere 10% chance here, for a total of only 20% if you send two of them together.
Last and least, there's the Friendship Arena and Pirate Wreck. The Friendship Arena gives you an 8% (total) chance of netting an Emerald with a full load of explorers (any will do; Clover has no special advantage here, so don't waste him on this place!). The Pirate Wreck gives you a mere 1% chance per explorer (unless you send Clover, who has a 100% chance as mentioned above), for a total of 4%. Yuck!