Personally, I voted Behemoth.
Behemoth feels like it fits better into the psuedo-Final Fantasy atmosphere and naming conventions alongside Ifrit and Shiva, among others. Behemoth has been used by Square Enix as gigantic monsters for some time now.
As a side note, micado commented that he felt that Colossus fit better because Behemoth was a monster in Christian mythology and not much else, but other creatures such as Ifrit, Gilgamesh, Shiva and Fenrir are all mythological figures out of different mythos as well. It's not as if Behemoth is out of place among the names or particularly over-religious in comparison to the others. Shiva is a not insignificant Hindu god, Ifrit are an Islamic elemental spirit that could be classified generically as "demons", Gilgamesh was an ancient Mesopotamian king and the central character of one of the earliest known works of literature the Epic of Gilgamesh, and Fenrir is a Nordic creature, a monstrous demonic wolf bound until Ragnarok encompasses the worlds when he'll be freed and join in the epic battle of the gods in the end of days.
For the TL;DR-ers: Behemoth totally fits in with the naming conventions of Square Enix used and the mythological names of the other creatures.