Deathwatch

The Deathwatch is the chamber militant of the Ordo Xenos and is composed of the most badass Space Marines from every chapter who are deployed according to their skills and which specific kind of xeno they are the most experienced at facing. If they have a devastator squad composed of ten heavy bolter equipped Space Marines who are sent to fight the Tyranids then you can bet that those heavy bolter equipped marines will be the best shots with a heavy bolter who know everything there is to know about fighting the space bug lizards.

Their armor is painted black save for one pauldron which remains painted in the chapter colors to avoid pissing off the machine spirit, surprisingly this paint job actually manages to look really badass even if it ends up being totally pointless with the Black Consuls and the Raven Guard. They get their shit done and get it done quickly and the only conceivable reason that they don't have their own codex is that they tend to be deployed in singular squads rather than fighting large scale battles. There are a few fan-made codexes for them as well as an RPG where they star as player characters though.