Definitly the D.

• Doom.

• Dune 2.

• Disco Elysium.

• Dark Souls 1, 2 and 3 as well as Demon Souls.

• Dwarf Fortress.

• Dark Messiah.

• Diablo 1 and 2

• Day of Defeat

• Dead Space

• Death Stranding.

• Deep Rock Galactic.

• Deus Ex.

• Dying Light

And that’s just what I have in my Steam library. There’s so many more D games that are also great.