Personally, I always found The Outsider his most poignant. I think it gets overlooked often because of his more sci-fi involved popularity.
And real quick, I'd just like to rant and say fuck Derleth. As a fan of Lovecrat's original, more nihilistic approach to the story, I frankly find the Cthlulu Mythos, though good fun, to be a bit of a travesty. Derleth took what was perfect cosmic indifference and made it evil. Why does morality have to exist with these creatures? Fuck that guy. I prefer the idea of them not giving a shit about mankind, and applaud Lovecraft's appreciation for our irrelevence.