Iron Man #55 is the first appearance of Thanos, one of the more interesting villains in the Marvel Universe. The excellent video below give his in-depth storyline. Iron Man #55 has always been a key Iron Man issue standing out from the crowd and the issue has enjoyed “sleeper” status. As of 2011 he was rumored to be an additional villain in the Avengers movie (see Avengers Movie Gets a New Villain?)
Make sure to check out the issues of Iron Man 55 for sale.