I'd have to say it was Black House. I've only been smoking for about a year, and after hanging around in here for a couple of months, I was curious about the "secret society" of Black Bloods that seemed to exist. I ordered a tin at the same time I ordered my first tobacco sampler, and when I tried my first bowl, I was completely unprepared... the scent stayed with me for days, the nicotine buzz was new (remember, former non-smoker here), and the flavor was amazing.
And then when I smoked the second bowl, I had one of those transcendent smokes that was just effortless - the pipe basically smoked itself down to ash without relights and with minimal tamping, allowing me to simply enjoy and discover the subtleties of the blend. I was hooked.
I've had "Aha!" moments with a couple of other blends - Escudo, Luxury Bullseye Flake - but that one was the one that got me.