I'm not counting thrash, otherwise it'd be RiB or maybe some Pantera. The first true extreme metal (with extreme vox and all) was some random shit I heard online when I should have been doing a french project in 8th grade. It was some site with random samples of obscure death metal songs (it wasn't a metal site, either, it seemed like one of those generated web sites tbh). I heard a couple songs on there that seemed awful cuz of the vocals, but there was one I really liked. I remembered it was called Twisted Tower of Fire, but I never knew who it was by. I forgot about it for a couple years until the other day I remembered it and searched on google. Turns out it was Twisted Tower Of Fire by Dark Army.
The keyboard intro is fucking awesome, aside from that pretty average, but I still like it. Doesn't approach the epic evil awesomeness it had for me the first time I heard it way back when.
The first extreme metal album I had was In Flames - The Jester Race. It was a blind buy based on the cover and the price ($5). Needless to say it fucking kicked my ass.