You are more than welcome to talk about Chaos as well, but I'm personally not interested in them, which is why I brought up only SM.
Well considering you can name only one chapter, i chose Sons because to me they are most affected by Chaos - unlike rest CMS who are, twisted yes but still themselfs(in matter of speaking). They are more army of revenants than corrupted humans and vibe of Ancient Egypt plays good role in it. TL;DR From Sons i get vibe of lovecraftian horrors(unfathomable evil), while rest CSM give me vibe some zombie apocalypse.
I personally adore the Salamanders by the way in which they value all human life unlike many other chapters. However, the Lions I believe are the best simply because they are the few who spoke openly out against the Inquisition as a whole, and were willing to take that risk with the fate of their chapter. Noble, just not very smart