(Inspired by u/Thunder-Bolt) My 5 favourite successors
[1.] The Storm Wardens: Insular Celt-inspired (far from just the "Scottish Space Marines" misconception that they're often labled with), this honour-driven warrior culture breeds marines who prioritise wisdom on the battlefield, extensive planning and intel gathering, and even strategic debates prior to taking to the field of battle. But once there, their zeal is obvious, as they speed towards the enemy in armoured transports, supported by artillery to cajole the enemy hordes into position, jumping out, taking their Sacris Claymores to the enemies' throats, and often challening their leaders and champions in one-on-one duels. They're mystery-chasers, too, hungry for more lore and revelation; driven at least in part by their history as a chapter whose first company was frozen in status due to mysterious and unspeakable circumstances. They're known to be particularly accepting of non-standard human variants (Ogryns and the like), which I like. knows for sure who they're a successor of. Some say the Iron Hands, it wouldn't be silly to assume it'd have been the Ultramarines, some even theorise that they're descended from loyalist World Eaters. Personally, my money's on the White Scars.
[2.] The Guardian of the Covenant: What more could you ask for in a Dark Angels successor than a chapter that looks even more knightly (with their mostly steel-coloured armour, and prominent robes), acts all that much more warrior monk-like (with the gathering of knowledge being one of the things that motivates them most) and even has a history of looking out for standard humans more than most.
[3.] The Dark Hunters: Do you like the savage but cultured White Scars? Me too. How about the fighting-smarter-not-just-harder Raven Guard, making use of shadow, surprise, and obfuscation as a force multiplier? Same here. Do you wish either/both of those had a less plain colour scheme? Well that's why these guys are great. They're a deep blue-painted White Scars successor who have adopted Raven Guard tactics into their combat style to a notable degree, after having fought alongside them in the past.
[4.] The Howling Griffons: I love space marines at least in large part because the concept of a space knight really appeals to me. Here, we've got an Ultramarines successor that takes that concept to its fullest (without becoming a religious templar). They're hugely honour-driven, taking their vows pretty much more seriously than anybody else, and even paint their armour in an almost Bretonnia-style of medieval artistry and boldness.
[5.] The Absolvers: Blood Angels successors who are, for a real change, quite different from the rest of their fellow sons of Sanguinius. Rather than feeling like yet another blood-thirsting noble vampire knock-off, with their own slight variation (which too many Blood Angels successors feel like, in my opinion), this chapter doesn't reveal to outsiders (and even many of its own chapter members) who they're a successor of; out of humility. They see Sanguinius as the mightiest and most worthy of people, so much so that they don't yet feel worthy of openly and brazingly declaring their affiliation. They instead characterise themselves with another vow and honour-driven motivation, a strict adherence to the Codex Astartes, and a presence on the battlefield that leaves on-lookers wondering they're anything from an Ultramarines successor to sons of Jaghatai Khan.
Some honourable mentions (because I cheated a little, above: I picked my top 4, and then the 5th entry - the Absolvers - were just another random one from the rest of my top 10/15 or so; chosen only because they have artwork like this, while the others don't):
* Black Guard
* Dark Krakens
* Destroyers
* Marines Errant
* Red Talons
* Shadow Wolves
* Star Dragons
* Tigers Argent