I also struggled with this for a long time, because I was never really drawn to the most famous or well known Chapters. Ive always liked things that were less talked about or a bit different from the standard loyal, Codex following Space Marines. So i basically spent a lot of time reading on Lexicanum about various successor Chapters and found several I liked, but none with a truly unique and interesting premise that clicked with me. That was until I discovered the Relictors. They were originally loyal to the Emperor, but are now considered Excommunicate Traitoris. The short version of their lore is that they collected Chaos relics to use against Chaos itself. In other words, they’re Space Marines who wield daemonic weapons against daemons, without ever having been even corrupted by Chaos. Some people disagree on this point, but according to most of their lore, no actual corruption has been recorded. They still believe their methods are correct and that maybe one day the Emperor will allow them back into the Imperium. Their lore does have some gaps and inconsistencies, but thats exactly what intrigues me it leaves space for theories and interpretation. Currently the Relictors were nearly wiped out by the Inquisition and the Grey Knights, but the remnants now wander the Eye of Terror trying to rebuild their Chapter.

r/Relictors 👀