First appearance of Iron Man, of course. My friend actually found a copy of it for me when we were way younger - in about 1985 - It was in pretty rough shape - but probably a 2.0 as I recall , and he found it in a comic book store somewhere for $40... Then years later it was stolen.
It's the one book that if I were to find an affordable copy I would snap up and never sell - but these days even a 2.0 is CRAZY expensive