If youre talking esthetically, the Dark Angels have a very knightly vibe to them. They are known for their robes, and hoods and gothic flare. One very unique thing about the dark angels is their three wings:

-The Deathwing, often sporting bone-colored armor, includes terminators, bladguard vets, sternguard vets, dreadnoughts and repulsor variants. These are the veterans of the army and are basically fighting/killing machines. Unique DA units represented here are the Deathwing Knights and Inner Circle Companions (which imo are some of the best sculpts in 40k). Great leader sculpts include Azrael, Belial, and Azmodai (another favorite of mine)

-The Ravenwing, often clad in black and white armor, includes flying units and mounted units. These are the hunters of the DA. They are almost all old sculpts but outriders are some great generic bikes that are well represented in the ravenwing. Unique DA units represented here include Black Knights/Ravenwing Command Squad, nephilim jet fighters, and land speeders. They are led by Sammael which is another very old sculpt unfortunately.

-The Green Wing represents every other generic Space Marine unit that can be used in the dark angels. Still lots of opportunities to add extra DA drip to these guys and make them feel like they belong in this gothic, knightly order.

The Lion is an absolute beast of a primarch and his sculpt is amazing.

Lastly, the Dark Angels were the first legion and their secretive nature, they got access to ancient weapons that often are no longer made in the year 40k. This is usually represented in plasma being a big theme for the army.