I've been into Ozzy and Black Sabbath since I was literally 5 years old, and Metallica since about 3rd grade, but besides for Metallica and Ozzy (I don't think they count, because EVERYONE has had some experience with them, being that they get so fucking much radio play...they are way too mainstream for me to consider them "my first metal encounter"), I will have to be honest and say...Fear Factory was my first real metal experience.
-------what follows is a little off topic, but...eh...read it if you wanna-------
Actually, before Fear Factory, I was getting into shit like Limp Bizkit and Deftones (aaaaa!!!...don't make fun of me!), but I don't consider them real metal. Fear Factory, though, considering their speedy double bass, and hard vocals, I consider to be real metal, although by my standards today, they aren't very good metal.
And very soon FF led me into liking bands like Pantera, Skinlab (I know, Skinlab is uberghey), Machine Head, and Pissing Razors (also uberghey). Somehow, without ever hearing those bands before, I had a gut feeling that they would be similar to FF, and lo, and behold...they were.
Next came Death, then by accident, one day, whilst out looking for another Death album, I accidentally bought the compilation CD called "Death...Is Just The Beginning Vol. IV". Apparently, I missed the "...Is Just The Beginning Vol. IV" part, and mistook it for a Death album. And then the rest followed...COB, Agathodaimon...etc.