The Blood Angels are still really good. In fact, they were so good for some time that GW had to put out a specific rule to nerf them. The red ones have access to the best 'Smash Captains' (Captain with jump pack, thunder hammer, and storm shield) as well as golden angels dudes, black angry dudes, and strategems that allow them to come in and hit VERY hard. That created the 'alpha strike' in close combat and basically destroyed many armies' chance of a fair fight against them. If you don't know, close combat has some very distinct advantages in the current edition of 40k- not the least of which is locking units so that they have to retreat (if possible) and not be able to shoot. Locking units in close combat is a game killer. Add to that the 'fly' special rule that all these jump pack dudes get and suddenly the Blood Angels were just too powerful for the meta.
Admittedly, the Blood Angels don't get such cool rules as the new Doctrines and their strategems are a bit outdated in many cases. But their chapter trait is the best in the game for power-armored armies- they get +1 to wound when they charge, get charged, or heroically intervene. Having Intercessors or those funny-colored guys with two attacks, armed with chainswords for extra attacks and then the new rule, Shock Assault, that gives an additional attack... You can see how ridiculous this is. Being able to wound Imperial (or Chaos) Knights on a 3+ with nothing more than a knife is really, really good.
On that note, the Space Wolves' chapter trait isn't much worse. They gain +1 to hit in those same circumstances. Now, the feral boys don't have alpha-strike units like the Blood Angels, but they do have a preponderance of storm shields, multi-wound beasts that move really fast (but, but, there are no wolves on Fenris!!!) and the ability of the copious amounts of characters to heroically intervene up to six inches. And the army is 'chaos-armed', meaning that they have the full suite of wargear that can give them all the attacks in every phase possible. I know that my boys-in-blue would LOVE to have bolters, bolt pistols, and chainswords on the bog-standard tactical marines, even if they can't have two special weapons... Moral of the story- Space Wolves aren't hurting that bad either.
So then we get to the Dark Angels. What's their deal? They're hunters. They're the army that runs around the galaxy hunting for some bad dudes based on rumors alone. They have massive amounts of speed at their disposal, from a moving Chapter Fortress in space to speedy bikes and deep-striking Terminators. This is represented in their chapter trait as... oh, wait. They get to reroll 1's if they DON'T MOVE. What? Oh, and they pass morale automatically or never lose more than one model for the rookies on the force. This does not at all in any way match up with the fluff. It helps Dark Angels players not have to worry about bringing Masters (Captains in other armies), but it also encourages them to bring static units like Devastators and sniper Scouts. But it gets worse...
The majority of the Dark Angel army is supposed to be made up of the 'greenwing', meaning Tactical Marines and the like. Instead, you see them running Ravenwing and sometimes Deathwing more often than not. Why? Well, simply because they're better than the greenwing. Getting to reroll ones doesn't match up to bikers with assault plasma guns or Terminators that can shoot the turn they deep strike while surviving awesome firepower in return. To make matters even worse, White Scar bikers are better than Ravenwing (Ravenwing get a jink bonus save when advancing (and giving up shooting for normal bikers) while White Scars get to retreat and charge) and the Deathwing have to pay at least two command points to take advantage of shooting as soon as they deep strike- and that's if you forgo close combat weapons for storm bolters! Oh, and both Deathwing and Ravenwing can get really expensive if you load them up to take advantage of their theme. Hmm...
There are some other, small issues as well. For example, Dark Angels have company veterans, but not Sternguard, so no special issue ammunition anywhere in the book. They also don't have access to Centurions or Vanguard squads, not to mention the other flyers. I DO NOT want Dark Angels to be green-colored marines with access to everything the Ultramarines have like the Blood Angels seem to be (but red), but I do think that sharing some of the technology would be in order. After all, Bobby G ordered Azrael to accept the Primaris guys, but he forgot the Centurion suits? Give the Dark Angels some of this stuff, for Emprah's sake!
How do we fix that? To me, it seems like a return to the Dark Angels' theme- hunters. Take away the goofy standing still rule, it just doesn't fit them. They aren't Imperial Fists! Keep the bonus rules against Chaos armies, but add something that actually matters against the other 95% of the game. And I'm just not sure what I would do to make this real. After all, I don't want to the White Scars or Raven Guard to lose their uniqueness, but there's got to be something. We'll start with the my point above- just give them Centurions and Sternguard Veterans. That'll be a great start (and a fantastic sales point for GW).
After that, my suggestion would have to focus on the chapter trait... First, I was thinking that the Dark Angels could lose the penalty for moving and shooting. But there's two problems with that- I feel like that's going to apply to the Iron Hands when they get their book and this still encourages the standing still build for Dark Angels. So maybe not that one. Then I thought about just applying this to just bikers and Terminators. But bikers don't need it and Deathwing don't really care. So what then?
I then thought about making this army the first 'fearless' army in this edition of 40k. But they're Space Marines and the power-armored champions rarely care about that mechanic anyway, especially with the And They Shall Know No Fear rule and running smaller units than horde armies. Plus, the Inner Circle rule already covers that and battle-forged armies can't lose more than one model on a failed check anyway. So maybe not that rule.
The last suggestion that I have would be... Hmmm, this is harder than I thought it would be! I even consulted with my son, who currently plays my Dark Angels and may have some insight that I wouldn't. After much deliberation, we basically came to one possible suggestion-
As always, Happy Gaming!