Among the Space Marines, the Salamanders are typically considered the most "good guy" of all the sub-factions. They actually live normal lives among the people of their planet when they're not fighting, so you get the feeling that they actually care for the people of the Imperium and aren't just warrior-monks completely up their own ass with visions of glory and battle or whatever.

The cool thing about 40K though is that you can make up whatever you want and claim that it exists somewhere in the galaxy, and nobody can contradict your canon because that's literally how the entire hobby works. My army is Sisters, but they're not based on any established convent and my plans are to get a bunch of Soviet decals to put all over them and say that they're a revolutionary vanguard, because "why not?"