Amigos del Mar. Even if you choose another dive op to start many of them will be contracting you out to Amigos anyway. This is a good thing. There are other boats that go but this dive is not the one to cheap out on. When you get to the island go to the ops that offer blue hole and get a look at the boat you'll be taken out on. Meet the staff, make a decision. But before you actually book or go out on the Blue Hole dive day, be sure to have done as many local dives as possible and preferrably testing yourself on depth. The Blue Hole dive is not a hard dive but it is certainly one you should be prepared for.