I have the Mk2. Got it back before the others were announced and was my only option. It's actually a beautiful figure, but like a lot of others in the end it doesn't feel bulky enough to represent War Machine; especially if you just want one representation. So I knew I would get either the MK1 or Mk3 when announced and sell my Mk2 (this will be the first time I ever have sold a HT figure).
I chose the MK3 over the MK1. It was a very difficult decision and took the Mk3 being waitlisted (the first time) for me to know which one I wanted. I got on the list, and it converted (then went back up for pre-order for a long time before waitlisting again- lol).
I like the MK1, I like the more classic comic look too, but in the end I felt the MK3 still had enough of that bulky tank feel and would look better with the my other Marvel HT figures. I thought the MK1 would look great by himself or with the IM MKVI (which I have pre-ordered), but I wasn't likely going to display him like that. The Mk3 feels like a finished product, ready to be an Avenger with the rest of them. It may not effect the look of the figure but something about the MK1 (according to the narrative of the movie) being a bunch of Hammer Tech built over top the IM MK2 also was hard to forget. The "Ex-Wife Missile" looks cool, but... "
Hammer Tech". My usual MO is to try to get the figure which reminds me most of the comics. I think it could be argued that that would be the MK1 when it comes to War Machine. Again, this was a very difficult choice, but in the end I felt I liked the MK3 better and it I think it still captures enough of that classic comic feel.
I don't think there is a wrong choice between any of these versions, even the MK2 (again, that is a beautiful figure too). But I can't wait for the Mk3 to arrive. I love how armed to the tooth the figure is while also being both bulky and modern. I also love the little details like the kill tallies, the Air Force symbol on the shoulder (even if not accurate) and "Col James Rhodes" printed on the armor as well.