There's five suits that can be unlocked in the game.
- You get the Silver Centurion armor for finishing the game. (If you're playing the PS3 version, you actually unlock the armor that he was in The Ultimates comics).
- You get the Extremis armor when you beat 80 of the crime syndicate baddies in the One Man Army vs. Maggia level. This is my favorite armor. It consists of a gold Undersheath, which is like a nano, second skin that goes under the second part of the armor, which is the External Suit Device, which is the red plating.
- You get the Classic armor by beating 80 mercenaries in the One Man Army vs. Mercs level.
- You get the Mark I armor for beating 80 corrupted scientists in the One Man Army vs. AIM (Advanced Idea Mechanics) level. I'm not a big fan of this grey armor. I prefer the red and gold look that he uses currently.
- You get the Hulkbuster armor for beating 80 corrupted scientists in the One Man Army vs. AIM-X level. The Hulkbuster armor looks really weird in my opinion, but it's actually an addition to the Mark I armor. It fits over it and was made to use against the Incredible Hulk if Stark ever met up with him again.
As far as hardest armor to unlock, I didn't think any of them were super difficult. It's just a grind to get to the 80 bad guys beat, but it will come with the natural progression of the game.