If you suggest any that I forgot, be sure to vote on 'Others'.
As much as there will be "how about this and that", if you didn't vote for them, your suggestion phails

Kong (King Kong)


Godzilla (Godzilla)


Clover (Cloverfield)


T-Rex (Jurassic Park series)


Alien Queen (Alien series)


Megatron

If I could edit the poll, I would have put in Marshmellow Man (Ghostbuster)

I had a thought on Megatron to be included or not.

If I didn't, people will be asking the opposite of your question:
"Why didn't you include Megatron? He's a giant monster, just a little smarter than the rest"

I voted Godzilla.

The huge blast from its mouth is more than enough to eradicate anything in its path

If measuring up the size according to each respective films, I'd say it'll be in this order (from big to small):

1. Clover
2. Godzilla
3. King Kong
4. T-Rex
5. Megatron
6. Alien Queen

Size does matter