Just to add a little more, the Amazons are indeed the creme of the crop of Earth (or elsewhere, I don't entirely remember how far that reputation goes), but each individual space marine is already the best of the best. Most chapters recruit from death worlds, recruiting the best of the best across scores of worlds and trillions upon trillions of human. They all have their trials, but an example is the Space Wolves, recruits thrown to the wilderness of fenrir, a frozen wasteland of a world filled with wolves bigger than men, krakens that sink ship, and moose that **** over armies, forced to crawl back hilw their body gets ****** over. These are the basic marines, the fodder. After all that you have the veterans, the leaders of small squadrons, or companies, or bigger seccions yet, the leaders of entire chapters or the best among the legions of old. Above all that was the likes of SIgismund, who even the likes of Abaddon thought he had survived merely due to how old he was when he confronted him. Kharn was the equal of this dude, before becoming a Chaos Marine, and before 10,000 years of slaughter and combat.
But I gotta agree in the precision department, at least right now because I can't remember any feats for such stuff for SM or Kharn. I'll have a look just in case.