Metal came in my life through In Flame's Clayman. I thought it was pretty extreme back then. I had never listened to anything heavier than Disturbed, so to a 13 year old it was as extreme as it got.
I continued working through In Flame's discography, from Clayman on up to Come Clarity, then realized most of the newer stuff was crap. Soon after I discovered Insomnium, which started me down the road to death metal. Then I kind of worked backwards from Clayman, trying Whoracle and The Jester Race.
Mainly I stuck to bands with lots of melody in them and lots of hooks. Other bands influenced me to move away from that. I think the first real extreme metal band I ever enjoyed was Sepultura's Roots, if Insomnium isn't 'extreme' enough for the purists.