Um, for all its flaws, I don't think I'd choose the Herbert West series (and remember, it was a series originally, published as such). I've read it several times and it has its odd appeal and even shows his concern for literary technique rather more than one might suspect. It also exhibits some of his genuine concerns, albeit in rather distorted form.
Another such, "The Lurking Fear", I would also not quite put in that category for much the same reasons, though it, too, suffers from many of the same faults.
On the other hand.... "The Street" would definitely be a contender for his worst original fiction (some of his revision tales would leave it far behind, however). It, too, has its strong points (to my way of thinking), but it is still among his worst. I am tempted to put "The Hound" in this category, save that I have become of S. T. Joshi's view that it is -- at least in the main -- a parody of his own writing as well as of the Decadents which he in many ways admired.
Then again, a fair number of his juvenile prose works (this is less true of some of his verse of the same period) are truly bad, even for a child. I do not refer to such things as "The Beast in the Cave" which, while a juvenile tale (written between his fourteenth and fifteenth years, if memory serves) actually shows considerable ability, but rather his earliest fictional writings.
A large proportion of his verse, if considered as poetry, is simply a failure. To me, it has other redeeming qualities and I can often enjoy it on different levels... but, save for a respectable portion of the fantastic verse and some of his satire (and an occasional piece of occasional or nature verse), it simply isn't poetry by any stretch of the imagination.
Some of his essays are rather dull and lifeless, but most of those are from series which he did for various newspapers, and required a lot of forced repetition. On the other hand, a not inconsiderable number are quite brilliant in one way or another, and some of his travelogues are gems, while some of his literary and philosophical writings show an amazing talent and acute mind.
Even a few of his letters make me wince... but here the vast majority are scintillating examples of this art form.
Several of his worst works are his revision tales, particularly those for Adolphe de Castro ("The Last Test" and "The Electric Executioner"), those he touched up for his soon-to-be wife Sonia ("The Horror at Martin's Beach", a.k.a. "The Invisible Monster", and "Four O'clock"), and the first of the Clifford M. Eddy revisions, "Ashes"... which may well (depending on how much of a hand HPL had in it) be counted as the single worst thing he ever touched....