The Blanched Soldier
(Although, having said that, I suppose I have to somehow account for The Illustrious Client, which is not only a later story that I actually like, it may very well be my favorite story in the entire Canon! Well. I never said I was consistent in my opinions.)
It's not that the solution to the mystery is howlingly obvious, or even that Holmes has such a small role in the story's conclusion. To me the real disappointment is that this adventure is the one Holmes relates himself.
Gone is our beloved biographer John H. Watson, except for sneering swipes Holmes takes at the man when he can't defend himself. Instead we have Holmes airily telling us that this time he's bowing to Watson's repeated "try it yourself" challenge – then admitting that he can find no way around telling the story in Watson's way.
So the story ends up being a pale imitation of a typical adventure, made worse by the unfulfilled promise of being told from the unique point of view of Holmes himself. Well, we can't have everything. I will admit that in BLAN you get an extremely atmospheric tale. And that alone makes it an enjoyable read, whether it is among the best tales of the Canon or not. Do you agree?