My favourites so far:
Mark 4 Secret Project because I really like the paint scheme and never thought that I'd get one given that I only started collecting Iron Man figures last year.
Igor because it's such an imposing figure and despite only a few seconds of on screen time, everybody remembers seeing that armour in the movie
Stealth Mark 7 - despite the stand problems, I think this figure looks amazingly good in hand.
Mark 3 Die Cast as the classic armour.
Iron Patriot as this was my first armour and the one that started me down this path.
To be honest, I like all of them but these five are the ones that I would least like to give up from my collection if I had to.