This is such a general question that we often have a hard time answering it. The answer really depends on you. What do you want to see? Do you want to see sharks? Octopus? Tuna? Turtles? Do you want to go spear fishing? You have so many options that narrowing it down to just one is often very difficult for people. For the experienced and adventurous divers, you have Isla del Coco while for the more relaxed divers they can head out to the Catalina islands.
There really is something for everyone. Get to know the country and talk to fellow divers before making your decision on where to dive in Costa Rica.