The following is a 224-word palindromic poem called “Dammit I’m Mad” by comedian Demetri Martin:

Dammit I’m mad.
Evil is a deed as I live.
God, am I reviled? I rise, my bed on a sun, I melt.
To be not one man emanating is sad. I piss.
Alas, it is so late. Who stops to help?
Man, it is hot. I’m in it. I tell.
I am not a devil. I level “Mad Dog”.
Ah, say burning is, as a deified gulp,
In my halo of a mired rum tin.
I erase many men. Oh, to be man, a sin.
Is evil in a clam? In a trap?
No. It is open. On it I was stuck.
Rats peed on hope. Elsewhere dips a web.
Be still if I fill its ebb.
Ew, a spider… eh?
We sleep. Oh no!
Deep, stark cuts saw it in one position.
Part animal, can I live? Sin is a name.
Both, one… my names are in it.
Murder? I’m a fool.
A hymn I plug, deified as a sign in ruby ash,
A Goddam level I lived at.
On mail let it in. I’m it.
Oh, sit in ample hot spots. Oh wet!
A loss it is alas (sip). I’d assign it a name.
Name not one bottle minus an ode by me:
“Sir, I deliver. I’m a dog”
Evil is a deed as I live.
Dammit I’m mad.
Source: Slate

This is by no means the longest palindrome ever written, but it reads much smoother than most. I am not sure how he wrote it but I can imagine the process would be quite difficult and frustrating. Below are some examples of other notable palindromes, but I maintain that the poem above is the best.

A man, a plan, a cameo…:
A palindromic list of 17,826 words compiled by Peter Norvig using a computer program that he created. My challenge for you is to find the middle letter in the list.

Dog Sees Ada: A 306-word poem by Gordon Dow. This poem makes much less sense than “Dammit I’m Mad”, and seems a bit of a stretch, even for poetry.

A Gassy Obese Boy’s Saga: A 5000-word poem by Will Thomas. It just rambles on after a point, but still quite impressive.