I'd like to just jump into the pit real fast and say that the original Ultimate Iron Man armor is the best design. It's the most believable for me, too, as it's large and bulky to house all the electronics that would be required in a suit like that, not just inch-thick bands of metal conforming to Tony's body. Plus it looks boss as fuuuccckkk:
Sadly, I'm pretty sure he just uses 616 Iron Man armor in the Ultimate comics these days.
Edit: Ok just did some research and WTF, talk about inconsistency between titles. in Ultimate Comics: Avengers, the covers show him wearing the Ultimate armor. In Ultimate Comics: Ultimates, it shows him wearing the normal 616 armor. In Ultimate Spider-Man, he's wearing 616 armor but with slight design cues from the Ultimate armor. Get your shit together!