And if ye are in doubt concerning that which We reveal unto Our slave (Muhammad), then produce a surah of the like thereof ... Quran 2:23
And this Qur'an is not such as could ever be invented ... Or say they: He hath invented it ? Say: Then bring a surah like unto it.... Quran 10:37-38
Or they say: He hath invented it. Say: Then bring ten surahs, the like thereof.... Quran 11:13
As you read these verses, you'll notice that the challenge isn't clearly stated. Will one surah that is better than the Quran's suffice (as in the 2:23 and 10:37-38 challenge), or must we come up with 10 superior surahs (as in the 11:13 version)?
Either way it should be easy to show that the Quran is not the revelation of Allah -- or any other god. There are plenty of atrocious surahs in the Quran.
Here is surah 111 (Palm Fibre) for example.
The power of Abu Lahab will perish, and he will perish.
His wealth and gains will not exempt him.
He will be plunged in flaming Fire,
And his wife, the wood-carrier,
Will have upon her neck a halter of palm-fibre.
And here is a surah that was written in response (I'm not sure who the author was, but I saw it first in Waterrock's commentary on Surah 10:31-52.)
In the name of Marvin, most-merciful, all-compassionate:
Damn both hands of my neighbor Sam; damn him!
His money and children will not save him!
He will be burnt in a blazing flame --
Sam and his dame, who is also to blame.
As she was carrying wood to her home,
She put some thorns in the path where I roam.
So she shall suffer a torment most dire,
Dangling in hell from a noose of palm-fibre.
And with that you should all be inspired.
(But those who participate should be aware of Allah's rules. Quran 2:24 says that you will burn forever in "the Fire prepared for disbelievers, whose fuel is of men and stones" if your surah is inferior to those in the Quran.)