I only really discovered 40K around the time of Dawn of War 1, and thus the MkVII is the Marine's Marine for me. The helmet is the most iconic part, naturally, but I also love everything else - the mixed armor bits, the weapons, the decorations, the ancient relics...
I also fell in love with their fluff of consumate warriors that can go anywhere and do everything by the virtue of their all-encompassing training (well, by the time you reach Tactical status), superhuman physiology, and powerful armaments.
Too bad the rules barely ever tried to reflect that and their universalist status has been increasingly lost as editions pile on.
Of course, I didn't notice that at first, as I had no previous exposure to miniatures, or miniature games, or military history. But today, my brainworms have drunk deeply of those other wells and my spirit rests uneasily.