I know this topic's a few days old, but as a huge sci-fi enthusiast and fan of power armor, I have to toss my two cents in.
I personally think that the Warhammer 40K power armor is more true to the concept of power armor codified by Heinlein in Starship Troopers, and by proxy, to the likely progenitor of the concept, being E. E. Smith in his Lensman series.
Iron Man is great, sure, and may sport the most widely recognized power armor, but it was by no means the first. Heinlein popularized the concept through Starship Troopers in 1959, as has been said, but even then he was building on a foundation set by Smith, whose work began in 1937. Iron Man himself didn't debut until 1968. I'm sorry if this has already been said, but I haven't thoroughly read the whole topic.
Anyway, my idea of power armor is that it is a widely used military tool, available at the very least to units of elite soldiers. So, in my mind, it's somewhat mass produced and not restricted to a single individual or handful of individuals.
Iron Man (and War Machine, etc.), conversely, from my admittedly limited understanding of the Marvel Universe, seem to be exceptions to the norm in their universe in that they are basically defined by their armor; it's unique, it's special. No one else has it, including the military. Though, I could be wrong about that.
As I said earlier, this contrasts with the power armor in Starship Troopers, Warhammer 40K, and the Fallout series, etc., where it is pretty much standard issue. There are grades and variants of it, sure, even exclusive models, but in and of itself, power armor in the aforementioned universes is just military hardware, used by many soldiers and not relegated to a few individuals, basically like the M16 used by US forces in Vietnam.
That said, I think Iron Man is cool, but I generally prefer the power armor in Starship Troopers, 40K, and Fallout. Those types of power armor are more iconic to me, personally. That's just my take, though.
Anyway, though it's more of a powered exoskeleton than power armor, my (sarcastic) nomination for most iconic power armor goes to:
Behold! The BGC-19, Joel Robinson's mobile drum kit from Mystery Science Theater 3000!