I would say that all of them were very awesome, but two of them really stand out the most. Make that 3. The first one was Mechagodzilla with this cool moves and agility, as well as his music. The second one I would say King Ghedora, with it's three heads and huge size. And the last one and my personal favorite Gigan with his buzzsaw in his stomach and hooks for arms. As well as his laser vision and especially having the best boss music in the game.

Gigan is one of my personal favorite monsters. I wish there were more movies with him in general. :)

Gigan is, hands down, the coolest of the Godzilla villains.  I know nothing about this particular game, but I grew up watching the Godzilla films. 

Oh, yes.  Gigan it is.  Mechagodzilla is a might overrated.  But, seriously, how can you top a thing with a saw in its torso?