The most important creature? It depends on what you see as important...
but I like to think "Cell" is one of the most important ones, since it's the basic structural, functional, and biological unit of all known living organisms and distinguishes the living world and dead-inorganic objects

Women all the life. Thanks God.

@Bahar - So you consider the cell to be a creature? Interesting. I've actually heard people say the Earth fits the definition of a living being, so I'd like to change my answer to "the Earth".

Maybe trees are)

@Haru:

1) I'm just pulling your leg.

2) The first few posters said things like "peace" and "happiness", so I wonder why you didn't grill them too...hehehe

So my real answer is, the dinosaurs. Big, cuddly lizards - can't beat them!