Since Iron Man doesn´t have a rogues' gallery with the same quality as Spider-Man or Batman, I tend to analyze his villains not for what they are, but for what they might have been. So my favorite villains are among those who have unfortunately been misappropriated, or fallen into oblivion.
Crimson Dynamo III (Alex Nevsky) - What a great potential wasted! Nevsky might have been Tony's equivalent of a Doctor Doom / Green Goblin villain.
Kearson Dewitt - Another potential Doctor Doom / Green Goblin combo that deserved to reappear. Technically he died, but there have already been characters who have recovered from worse death scenes.
Dreadknight - Another one that had great potential and is a pity that has been turned into a joke.
Techno-Killer - From Marvel Comics Presents # 132. Harold Marks is a guy who impressed Tony with the armor he created and even better, he didn’t have to steal Iron Man's technology to create it.
Jason Beere – From Avengers I#169. Armored industrialist invertor, one of Tony's main competitors, who could not stand the thought that the world will continue after his death and tries to destroy the planet. How such a cool character remains in obscurity, I don’t know.
Merlin - Meredith No Surname Given, member of the Circle from Iron Man Vol 5 # 2. Another potential Doctor Doom / Green Goblin combo that deserved to reappear (It seems like a pattern of preference is forming here).
Conner "Rebel" O'Reilly - From the Heroes Reborn universe, currently hidden somewhere in the original Marvel universe. Jim Lee and Scott Lobdell used a lot of Reed / Doom dynamics on this character, which turned him into one of my favorites way back in 1997. (Yes. We sure have a pattern here).
The Crusher - The original Crusher, was the first villain Tony believed he needed to kill. He would have been a Hulk without Banner’s redeeming qualities.
Skullcrasher - From Marvel Super-Heroes III # 9. A mix of the Ghost with Titanium Man. Impossible not to like.
Now, the three exceptions to this rule of mine:
Justin Hammer - Since Tony is a mix of Howard Hughes and James Bond, Justin Hammer is the perfect Marvel version of a Bond villain.
Madame Mask - Tony's version of Catwoman, just much more tragic and insane.
The Ghost - Yes, he may be a one-note villain, but he's a fanatic who never gives up and plays very dirty with absolutely no compassion.
Quite a lot of your favorites are armored. That's very interesting to me. I would normally limit the number of Iron Man rogues who wear armor. Would you like a world where all of your armored favorites were active regularly?
I too wish Dreadknight had never been turned into a joke. An evil knight on mutated horseback really does seem a cool opponent for Tony.