I'm very happy with my MkIV - it took a long time to decide But for me, he sums up the "complete" Iron man feel the best.
With the helmet on he looks serious, dangerous. And you can pose him to that effect..... but pop the Tony Stark head on, with the glasses and doughnut box, and the whole persona changes. A very versatile figure.
Having said all that I've got a MkII armour unleashed on the way 'cos I love how that silver suit looks and although Iron Man, for me at least, will always be red & gold, the silver sure is purdy! With all the extra parts you get for the price I paid, it's a steal.
Think these two will cover all my IM needs.
.......although. That MkV is pretty cool....... and a Warmachine would add some variety.........MkI is pretty awesome...........Aaah! Someone take my credit card away dammit.