Michelinie and Layton are the no-brainer choice for best of the best. They defined Tony Stark as we know him today. O'Neil's slow burning plot is the definitive drunk Tony arc and honestly should have been the last time any writer touched on the alcoholism because they always come off as trite in comparison. He brought Tony to his lowest point and built him back up stupendously with Iron Man #200 being one of the most satisfying redemption stories ever.
I never liked John Byrne's Iron Man run much. All he did was beat Tony down without building him back up and left a huge mess for Len Kaminski to clean up. Thankfully he was up to the task and Len Kaminski's woefully underrated run is one of my personal favorites. He understood Tony not just as a playboy billionaire but as a scientist, skeptic and businessman in ways that most writers don't manage. Unfortunately a lot of editorial messing around with forced guest appearances and other nonsense he had to write around weakens the run a bit but it's still excellent overall and when I think 90's Iron Man I think Len Kaminski and the Kevin Hopgood designed Modular Armor (not to mention the War Machine and Hulkbuster).
Kurt Busiek also did a great job of picking up the pieces after The Crossing and Heroes Reborn. Once again some editorial meddling and Busiek's own health kept the run from being everything it could have been but it was a return to classic form for Iron Man and well worth reading. The hardcover Omnibus is excellent and includes his awesome "Iron Age" retro story.
I agree that the Knauf's were underrated as well. They had a really strong showing after a long drought of mediocre and badly characterized stories. I wish they would have stuck around longer.
I have mixed feelings on Fraction. I was on board through World's Most Wanted before I just started feeling fatigued by his writing style. I think the guy is great at weaving plot-lines but not so great at writing characters.