So, I've recently enjoyed skimming through C. S. LEWIS: ESSAY COLLECTION AND OTHER SHORT PIECES, ed. Lesley Walmsley [HarperCollins, 2000]. Apparently published only in the U.K., this is a massive (894 page) collection that brings together everything from his short fiction (e.g. THE DARK TOWER, "Forms of Things Unknown", &c) to all kinds of ephemera: letters to the editor, pieces written in response to articles he'd read in some journal, and the like. Naturally, out of such a mass of material (135 separate pieces), the quality varies enormously. But I was struck both by how readable the whole collection is and by how far off the mark Lewis could sometimes be. This division is evident in his longer works, of course -- for every SCREWTAPE LETTERS or DISCARDED IMAGE there's also a MIRACLES or ABOLITION OF MAN -- but somehow it seems starker in his shorter pieces. Hence I thought it might be interesting to look in more detail at some of his worst essays, since his best work is so well known as to need no analysis.
Second Runner Up: "Why I Am Not A Pacifist" 
To be fair, I shd note that Lewis never attempted to publish this piece; it survives only because his friend George Sayer requested a copy and, many years later, provided Fr. Walter Hooper with a transcript. It was first published in THE WEIGHT OF GLORY , itself an expanded edition of the 1949 book TRANSPOSITION AND OTHER ADDRESSES (which had included five of the latter book's nine pieces); see page 18 of Hooper's Introduction to that book. Thus it is a better representation of Lewis as a debater than as an essayist.
Delivered to a pacifist group at Oxford in the early part of World War II, this one made it onto my short list not so much because I disagree with him (which, of course, I do) but by how weak I found his argument. That is, I read the piece knowing full well that Lewis was no pacifist, as his service in World War I and the actions of his characters in his novels clearly show. But I expected him to make a good case against pacifism, and sought out this piece because while I didn't expect him to sway my own opinion (I don't think I've ever changed my mind on any subject because of anything I've read in Lewis) I wanted to see what he had to say.
In brief, since it's a long article (more than a dozen pages in this densely packed anthology) and a complex one, I was surprised by the degree to which his argument hinges on an appeal to (civil) authority and majority opinion. He denies that individual revelation (what he calls "intuition") can be taken seriously when it goes against the example of most political leaders (in whom he includes the fictitious King Arthur!) and many of his favorite writers. Thus, for Lewis, belief in pacifism would put one in a minority, and on that ground alone is probably wrong. This seems to me such a strange argument that at first I thought he must be joking, but it's clear he really is arguing that the majority position must thus be the correct one ("the universal opinion of mankind"), as if morality were simply decided by majority vote (in which case, what about "give us Barabbas"?).
This alone of course would not make this essay rank among his worst, of course. Much more troubling are his tactics. Like a speaker more concerned with winning a debate than arriving at the truth, Lewis very carefully works to exclude all evidence that counters his point. For example, he quotes The Book of Common Prayer in support of Anglicans supporting 'a just war' in good conscience, wh. is fair enough, but then goes on to imply that ALL Christian denominations support 'the just war' --which is of course flatly untrue: cf the Quakers, Amish, Shakers, Mennonites, &c., not to mention the early church before Constantine got his hands on it. But what he actually says is "All bodies that claim to be Churches -- that is, who claim apostolic succession and accept the Creeds -- have constantly blessed what they regard as righteous arms". Can he really be claiming that if you're not a member of such a 'Church', you're not really Christian or your opinion doesn't count?
Finally, he does tackle the most difficult point of any Christian who is not a pacifist: the 'turn the other cheek' passage in the Gospel. Lewis's solution is to argue that this cannot possibly have any military application, since "the audience were private people in a disarmed nation" --which is disingenuous, given the long string of uprisings that bedeviled the area, the most well-known of which are the Maccabees' revolt a century before and the disastrous revolt a generation later that ended at Masada. He goes on to assert that Jesus would surely approve of anyone with rightful authority using force to defend himself against violence, most amusingly using as one of his examples a tutor fighting back against a student who wanted to hit him (just how heated did CSL's tutorials get?). He concludes this section with references to two New Testament epistles, both from passages urging submission to civil authority: ROMANS Chapter 13 ("Let every person be subject to the governing authorities. For there is no authority except from God, and those that exist have been instituted by God. Therefore he who resists the authorities resists what God has appointed, and those who resist will incur judgment") and 1st PETER Chapter 2 ("Be subject for the Lord's sake to every human institution, whether it be to the emperor as supreme, or to governors as sent by him to punish those who do wrong and to praise those who do right"); the latter comes from the same section that admonishes servants to obey their masters (long used as a justification for slavery) and for wives to "be submissive to your husbands". I shd have thought it much more apropos to cite the scene in Gethsemene where Jesus rebukes Peter for striking the guard (an example of violence in defense of the innocent if ever there was one).
So, in conclusion: Lewis was proud of his oratorical skill, and rightly so; he was by all accounts a marvellous speaker, and the few surviving bits of audiotape bear this out. But his gifts fail him here, leaving him with an overly complex essay marred by several ad hominem attacks on his audience in passing. Far better as a presentation of Lewis's ideas about the duty of submission to civil authority is "The Conditions for a Just War"  (pages 767-768 in the same collection), where he argues that it's not the hangman's job to decide who is and isn't guilty, merely to hang those sent to him for hanging; so too he claims that a citizen can't know whether a given war is just or not and so must leave that to his government, who alone can make that decision, and serve as a soldier in whatever war his government deems necessary. But even here he complicates his point by adding that the hangman "must not hang a man whom he knows to be innocent", nor the soldier "murder prisoners or bomb civilians", which acknowledges individual responsibility based on individual judgment after all. Even so, this short, focused piece is far superior to "Why I Am Not A Pacifist".
current audiobook: LOST CHRISTIANITIES by Bart Ehrman.