Search your feelings you know it to be true. OrdoThanatus is not responsible for your sick perverted sexual lusts. Imagining yourself with this Astartes is punishable by death by your local commissar. Have a nice day.
Hypothetically speaking, the Primarchs were made by taking the Essence of Manly from the Emperor Himself and then distilling it, producing Concentrated Essence of Manly in (roughly) Human Form. Short of invoking the Holy Trinity (Kamina, Simon and the Holy Buta), they are about as manly as one can possibly be... God-Emperor damn it, even Fulgrim is so Manly that Chuck Norris feels like a six-year-old girl (complete with pink princess costume and toy unicorn) in comparison.
I dunno. Canis Wolfborn is way up there too... but I think Leman Russ wins hands down. Unless primarchs don't count. (Dear local commissar, Do primarchs count? Sincerely, Asking about a purely theoretical scenario.)
I think the Black Templars are even more metal. They chain their weapons to their wrists and go on an endless crusade. Also they don't give a flying fuck about Rowboat Girlyman's Codex Astartes, and their chapter have way more members than they're supposed to.