Dante
Commander Dante is the oldest living (non-Chaos, non-Primarch) space marine who is not in a dreadnought. No one outside of the Blood Angels inner circle actually know how old he is. All that is confirmed is that he has been the Chapter Master for at least 1100 years. Even Logan Grimnar can't remember a time before Dante. He has kicked the ass of uncountable xenos, mutants and heretics in countless wars upon countless worlds. He is the victor of countless campaigns and his deeds are indistinguishable from legend.
In the plot, Sanguinius made a prophecy that there will be some great battle that will kick the asses of all others and that there will be a great, golden warrior standing between the Emperor and the darkness. Dante thinks that this golden warrior is him because he has an ego so massive that if it was a physical object it would collapse under its own weight and turn into a black hole from which nothing but douchebaggery may escape. In other words, it is about 10% the size of Rowboat Girlyman's.
He also worked with a Necron army to defeat a Tyranid hive fleet and is secretly gay for a Necron Lord named the Silent King (if you can be gay for a genderless robot, enjoy your Black Library).
Sadly, this article has nothing to do with the titular writer of Dante's: Inferno. Just because you wrote the first self-insertion fan-fiction does not mean that you get your own article on /tg/. Though he did inspire the modern version of hell, which lends itself to Monstergirls and the Warp.