Currently we really don't have very much information on any of these mobs. We know rouhly what they're meant to do, and where they live but thats about it. The specifics of thier spawning, mechanics or drops will greatly change how good each one actually is. Except D, because I don't think there is that much room for interesting mechanics there. Its just a big blaze.
For A, how common is it? Will they be infesting the ocean? Will they be really rare? Building anything underwater could be made extremely hard with a giant squid dragging you around and trying to eat you, if it ends up that there are lots of these it would be very annoying to do this. However mob A would make an exellent fish tank mob.
B has much the same sort of potential problems. Do they spawn regularly? Will they track players from long distances, unlike most other mobs which freeze if they are far away from a player? Seeing as ghasts don't do this I see no reason why this new flying mob would freeze mid air. How many nights do you have to skip to get them chasing you? Will they spawn in mid air (Personally I think they will because it would kinda defeat the point of a sky mob if they had to spawn on the ground) or on blocks? Will they announce thier attack with lots of noise or will they sneak attack you off the top of a skyscraper?
C is an interesting one, hiding under the ground is something we've not seen before. Unfortunatly the fine specs of the enchanting mechanic it uses are not known and heavily influence how useful this mob is. Perhaps it will be only useful as an undo button in enchanting if you make something too expensive to put more stuff on? Maybe it will remove a random enchant which would make it awful.
Currently i'm coming down on the side of mob B as I think it could potentially be the best if done well. Of course it could also be awful. A is fairly defined on what it'll do and would be irritating when building underwater. C I cannot see removing enchants ever being paticularly useful. Perhaps the case of making something too expensive by putting the wrong enchant on it, but thats about all I see being greatly useful so far. Oh and removing cursed enchants assuming it'll be able to do that. Which it might not.