Orkz. Orkz are the superior species. They're literally genetically designed to give absolutely 0 shits.
Massive casualties? Fine.
Demons raping you? Fine.
Watching all your friends twisted into a writhing, shrieking mass of flesh and tentacles? Fine.
Literally nothing phases these bastards. They walk where mere men crawl due to their sheer mental resilience. That which would drive you or me batshit insane wouldn't even bother an Ork.
Not to mention they literally accomplish shit even the Imperium can't through sheer fucking stubbornness!
What's that? My Kopter design isn't aerodynamic or physically capable of flight? Well fuck you, I says it is and it is!
Plus they're just plain fun!