The Dark Angels are important because they were the literal first Legion. They were the testbed for every other Legion, hence them specializing in basically everything from void warfare, bike spam, and massive tank formations. The reason why the Dark Angels are so good is because you can turn to them and get the traits of most other legions.
It's why they were previously tied for the title of most powerful legion with the Ultramarines, they were fucking huge and could adapt to virtually any threat. The only reason why they sunk was a combination of sacrificing themselves late in the Great Crusade against a great Xenos invasion and Caliban turning out to be a slumbering daemon world.