alright, let's decide what King of Beasts mean.
The main meaning is "godless dwarf-killing machine". But do other things that killing power matter ? Can a king of beasts be vanishingly rare yet still be a KoB ? Does a King of Beasts become more of a King of Beasts the more versions it stays lethal ? Can a king of beasts be on very specific biomes, such as evil oceans only ?
GCS is definitely the underground king of beasts. It kills dwarves with such ease, slaughters whole squads of melee militia with it's web attack and it can fight even with most megabeasts. Also, it has been lethal since 34a, and since a few versions you will encounter one at least in most playthroughs if you breach the caverns. It is also very much part of the established culture of DF since a long time, like carp and elephants.
so GCS = doubtlessly underground king of beasts. One can make a case for cave dragons, i don't know about them really.
Now let's try something else, the giant badger congas. In 0.31 (IIRC) it was so overly deadly AND common that it regularly killed fledgling fortresses.
Right now they are exactly as deadly as they were beforehand, but due to large amount of animals they are far, far less common than before. Are they still King of Beasts ?
Now let's try giant keas. Giant keas were previously -common, -attracted to your fortresses, -started fighting easily and killed dwarves.
Now with the new 0.40 combat mechanics they tend to be far less aggressive and therefore far less dangerous. They are still as annoying as ever, but they are no longer deadly. Are they King of Beasts ? (They don't have any competition in the air, though)