Can't remember my first.
Think I had a bunch of copied tapes to start off with.
I remember renting Among The Living from the library very early on, that could have been the first original tape I had in my possession.
And I have a very vivid memory of buying Rust In Peace on tape when it came out, guess that could have been the first one I actually bought.
When I was 11 (in 1991) I bought Blessed Are The Sick, and then proceeded to bury it in the back garden out of Catholic guilt. True story.