I first heard of it back in grade 11, c.5 years ago. My buddy who didn't like anime told me about an anime series he saw the first episode of, and loved. What he described it as, a bunch of girls going to school, and one is really smart or something, didn't convince me. Another buddy of mine had the whole series on disc and we got together for a viewing. Before that we'd been watching Evangelion. I wasn't terribly impressed, though intrigued.
Hooked up a laptop to the TV and began a marathon session that had me watching until the next morning. I loved it - the characters, the art direction, the way the story was written about regular life. Made a kind of connection with me, since I was around the same age as the cast.
I stayed mindful of it for some time after; Downloaded the soundtrack, did an internet art crawl (back when I used to visit YTMND) and even worked some of Azuma's faces and other stylings into my own artwork (though they don't show up so often anymore). Back in 2008 bought the Omnibus collection of the manga, just before I heard of the new work Azuma was doing with it. Anyways, this was the cartoon that led to me watching other Japanese series, and accepting anime as an art form.
TL;DR: the Anime that got me into Anime.