Admit I'm biased against Hume. Something about his Hard Scot style that left me as clammy as his pronouncement that "man was no more significant to the universe than an oyster."
For me, it remains Descartes Ontological Argument that provided the philosophical lantern in our collective cave of existential darkness. And I still see echoes of its pure logic and simplicity reverberating into the present. In Boltzmann Brains. In the intersections of quantum information theory and thermodynamics. Yes, even in the realms of design thinking, creativity and startup engineering...