First Runner Up: "Evil and God" 
Unlike "Why I Am Not A Pacifist", this little essay (only three pages long in the ESSAY COLLECTION) was definitely written with publication in mind, being a response and rebuttal to an essay by the celebrity philosopher C. E. M. Joad called "God and Evil"* It was also written quickly, since Joad's piece appeared in the January 31st issue of THE SPECTATOR and Lewis's reply in the February 7th issue. As such, it shows Lewis's facility of writing quickly and to the point. Also unlike the anti-Pacifist essay, here CSL's piece is well-organized and well-focused.
The problem comes with the blinding oversimplification of Lewis's argument. His chief concern is to reject dualism -- the idea that there are two equal powers in the universe, one good and the other evil. CSL's first objection is to state that he can't visualize two beings without some kind of framework or background creeping into his mental image, which he claims means both must be derived from some unpresented original. Unfortunately, his confession does not so much disprove dualism as testify to an astonishing failure of imagination on his part. Perhaps he shd have asked Barfield and Harwood, two of his closest friends, whose Anthroposophical beliefs were deeply dualistic, for help here in overcoming this hurdle. Even better wd have been someone showing him the yin-yang symbol, which presents dualism more comprehensively than Lewis's failed anthropomorphic attempt cd ever do.
The second objection, however, is where the real problem comes in, the point which entitles this essay to consideration as one of CSL's worst. Lewis argues that a dualistic religion is untenable because if Good and Evil are coeval -- that is, if neither preceded the other -- then we can't know which deserves our allegiance. In short, he argues that Good is better than Evil because it came first; Evil is bad because it's the younger of the two.** Otherwise, he claims, evil wd not be "parasitic" but have "the same kind of reality as good", making our allegiance to good simply an act of random partisanship.
That seems to me about as silly an argument as it's possible to make in a theological context, a logical and conceptual flaw of the first order. Lewis wd have done well to have paid more attention here than a brief mention in his final paragraph to the Norse concept*** (admirably summed up in Tolkien's "The Monsters and the Critics") where the gods lacked omnipotence: evil preceded good and wd eventually triumph over it, but that unfortunate fact only made our allegiance to the forces of good all the more important, since a heroic defeat cd mitigate the foreordained disaster. In short, his inability to conceptually grasp dualism forces CSL to conclude that anyone who accepts dualism is a lazy thinker who refuses to move on to what for him seems the logical next step: a bigger god behind the scenes.
This essay is emblematic of the overall logical weakness of Lewis's apologetics. This is ironic, since Lewis prided himself on his philosophical training and abilities as a logician, never realizing how poor the logical component of his arguments often were -- similar sweeping assertions and logical fallacies fatally undercut the central argument of MIRACLES , for example, as fellow Xian G.E.M. Anscombe pointed out, much to Lewis's embarassment. Tolkien affectionately described how much fun it was to watch Barfield (who himself wrote a number of Socratic dialogues) skewer CSL's arguments (he also later stated that Barfield's memoir of their mutual friend got nearer to the truth than any other he'd seen). And anyone who reads Leon Adey's account of CSL & OB's years-long philosophical debate, 'The Great War', is likely to come away feeling that Lewis was entirely outgunned and outargued on all points.
Where Lewis really excelled, whether he realized it or not, was in his use of analogy to drive home his point. This is one of the things that made him Oxford's best lecturer: he cd take a complex or abstruse subject and put it in terms anyone can relate to. Examples abound throughout his work (including his fiction and especially his literary criticism), and it's one of the things that make him so readable even now, forty, fifty, sixty, seventy years later.
So, this minor essay addressing a major point serves as a good example of something fundamentally wrong in CSL's work as a whole, hence its selection here as among his Worst despite it's being clear, straightforward, and readable, even enjoyable n/
...... I think it's fair to say that the degree to which his works succeed or fail is often specifically tied to the degree to which they rely upon logic. The more he attempted to rely upon logical arguments, the less successful the work; the more he relied instead upon analogy to make his points, the more successful -- although the latter strategy had its limitations as well, as we'll see in the next and final essay of this series.
current reading: AN [ILLUSTRATED] AUTOBIOGRAPHY by Anthony Trollope [1883/1987]
[[*Nt: Joad published a book of the same title in 1942, which I assume built on the ideas outlined in his 1941 essay.]]
[[**Nt: My wife observes that this is an odd position for a man with an older brother to take, but so it goes.]]
[[***Nt: Or, for that matter, the history of the universe presented in Pope's DUNCIAD wherein chaos proceeds order and our hard-won present-day order will one day lapse back again into "Chaos and Old Night".]]