Which armour mark is most associated with Iron Hands between Mk2, Mk4 and Mk6? I know Mk3 has some strong associations but I dislike the new kit and also prefer to limit Mk3 to specialists. I wanted to keep a majority of my troops and Commanders to a single Mark for that mass-produced feeling.

Mk2 looks closest to Mk3 so probably the most cohesive appearance.
Mk4 is probably the fluffiest for early Heresy? I assume Horus liked Ferrus because he wanted to convert him, and also Medusa would be able to produce their own suits.
Mk6 is explicitly the most popular suit late-Heresy and again the manufacturing capability, but it doesn't feel like it matches the IH MO.