u/Thunder--Bolt

Raptors: So my initial favorite chapter in the setting when I FIRST got into it was the Imperial Fists (tbf I still like them, but my favorite legion is like a toss up between them, Sallys and Dark Angels). Anyways, when I first heard about the Raptors I was immediately intrigued, and the more I read about them the more they became my absolute favorites. The camo usage, the guerilla warfare, emphasis on marksmanship and ranged combat, guard collaboration, and generally just an ability to work well with any imperial force because they hold no value in honor and glory; only completion of the objective is a worthy tribute to the Emperor in their view, and I love the hell out of that.
Star Phantoms: Possible Dark Angels successors, though the DA's deny the lineage, I like the Star Phantoms because they have a very interesting chapter cult, or the 'Mortuary Cult' as it's called. They hold the dead in higher regard than those among the living, and embrace the divinity of the God Emperor of Mankind as "Imperator Mortifex" -- judge of the souls of the dead and the keeper of martyrs. Their funeral rites are described as strange and elaborate. They see themselves as divinely ordained killers, and are consumed by the task of bringing death to those that defy the Emperor's will or seek to thwart His holy purpose. Also, they're combat doctrine is just cool as hell. They'll always take the biggest guns that put out the highest damage, have a particular penchant for plasma and melta weapons, and have little to no regard for collatoral damage. This is not the chapter you send if you want a planets infrastructure standing. Their whole Mortuary Cult thing is fascinating, and I like them for how unique their culture is.
Red Scorpions: I mostly like the Red Scorpions for being bros with the Raptors, but I think they're whole obsession with their gene seed purity is pretty funny. Beyond that, I like they're vibe. No nonsense, honor bound. They're a space marine's space marine.
Covenant of Fire: These guys are kind of funny because they seek out knowledge of Imperial lore wherever they can find it... but also burn and purge anything they deem heretical. Devout followers of the Promethean Cult. Interesting chapter that I hope gets expanded on in the future. Sick name and paint scheme.
Crimson Fists: To be perfectly frank, I don't know a ton about these guys. All I know is that whenever I see art of them or see them in any context I think 'Yeah these guys are fucking sick'. They are absolutely vibed tf out. Also they're chapter master looks cool as hell. Honestly I love these guys based on vibes alone. They're just cool, that's it.

My Top 5 Successor chapters as a relative newcomer to WH40K: