There is a quite simple argument to use against Creationist, i.e. someone who believes in the literal truth of the creation of the world according to the first chapters of the book of Genesis in the Bible.
Warning: the argument may rock your faith if you are a biblical literalist!
More on the flip...
Most people don't realize it, but there are two (yes, count 'em TWO) different and contradictory stories of Creation in Genesis, the first book of the Bible. The first story runs from
Genesis 1:1 thru Genesis 2:3; the second story picks up at
Genesis 2:4 and runs to the end of the chapter at Genesis 2:25.
In the first story, Creation takes six days and man (and woman) are created last after all the plants and animals are created. In the second story, Creation takes one day, man is created first, then all the plants and animals are created, and finally woman is created.
The two stories are posted side-by-side for easy comparison on this web page - check it out for yourself. And the next time a Creationist tries to foist some Intelligent Design poppycock on you, ask them if the world was created as described in Genesis; then ask 'em "which version?"
Warning: Most Creationist's faith is not actually very strong - learning that the Bible has serious internal contradictions may lead to their ill-being - use this knowledge with caution!