I love everything about the Astartes, them in general, the specific chapters you mentioned in the poll, their history, the Grey Knights, Death Watch, the days of the Legions and the Primarchs, the mind boggling feats they can perform, and just their sheer epicness in general.
But for all that, in a universe full of Xenos of all varieties, from the endless, inevitable Tyranids and the enigmatic and dying Eldar to the brutal and consistent Orks. Amidst the horrors of Chaos and the warp, unleashing deamons, traitor marines and all their other tricks. With all this and so much more, you have ordinary men, slugging it out against all of it. The Imperial fething Guard. Big god damn heroes.