Invincible Iron man vol 1 by Matt Fraction (5 nightmares) then the rest of the volumes.

what gambit987 said, you should also check out some of the stad alone storys like iron man the end and iron man vs whiplash

Extremis was pretty good.

I bought a really bad one to start off on Iron Man comics this summer, it sucked hard! I didn't realize it but it meant to go with the first movie. It had nearly the same dialogue and picture frames as it did in the movie, only it was a little more chopped up and seemed to skip parts. The only thing I really got out of it was what happened in the background with the Avengers and a few pages from an old Iron Man comic that seemed to skip some important things. Thanks for posting this thread, I'll be benefiting from it too! haaha.

" Invincible Iron man vol 1 by Matt Fraction (5 nightmares) then the rest of the volumes. "

I pretty much agree with what has been posted here already. 
 
For a trade I would go with Extremis. There is a new HC edition coming out later this month with the motion-comic adaptation included on DVD. 
 
For a volume I would start with the current series by Matt Fraction and Salvador Larroca, starting with the Five Nightmares arc and continuing on from there. 
 
And if you want some classic Iron Man then pick up Demon in a Bottle and/or Armor Wars. 

if you have the iPhone/iPad app, I think the first issue is free or was free of Extremis