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.
This is basically the driving force behind me attempting to write my own 40K ruleset where Marines are Good and thus Very Few On The Table.
Granted, this is going very slowly and has already taken around two years, which included "yeeting everything I've written myself and restarting by mushing Chain of Command with Bolt Action"
But I want Good Small Marines where you have only a handful of them, but they excel by breaking every rule in the book. In fact, your base ruleset should have normal human factions obey the ruleset as is, alien/weird factions break a rule each, and Space Marines break all of them, and Custodes shouldn't even be on the table.
A good idea in the new 30K Zone Mortalis ruleset is that you bring in a 2000 point list, but you're unlikely to field it in its entirety. This is a great idea, almost a backport of Chain of Command support options (in CoC, the base army is always fixed, but then you get to buy support options based on the strength differences between your force and the enemy's), in that you can bring more toys to the game, even if you won't get to play with them every time.
Who knows, maybe the situation on the table will turn in such a way that you'll find a good reason to bring in a unit you don't use often, but which has some cool models and lore.
Another reason to make your own ruleset is to make the iconic weapons of the game - bolters and chainswords - matter. For the longest time, Bolters were Rapid Fire 2, S4, AP5, which allowed them to ignore armor on chaff units (Orks, Guardsmen). S4 and AP5 made them different from Lasguns, which were S3 and AP-. But 8-10E rules went for the saner, better AP modifier rules, leaving bolters (smoll Marine bolters) without. At this point, they're just S4 lasguns, which is a damn shame when we're talking about .50 cal gyrojet SMGs firing explosive ammo.
Chainswords - chainsaws but balanced for parrying, as one website once put it - haven't had special rules for year. 8E-9E didn't do much for them, starting out with making them the same as having no weapon, but more attacks. In 10E, shit is going a bit wilder, but it's still mostly more attacks, maybe S4 sometimes? Not very evocative for sword that you can stick into a stinky heretic AND then do some real damage as revving it up.
Anyways, it's a small rant, and if you take anything from it, it's that you should give me money to buy the kits above + the Tortuga Bay bodies to bring them to True Scale.