Honestly, it would have to be Halo: Reach's art style for me. The way everything looks battered and worn, with a muted, wartorn sort of palette....the rain in the main menu....
The Spartan armour designs are especially good, man. They look the way Spartan armour should be; not bright and flashy and showy like Halo 5; but with those more muted colours (better for hiding, duh!), the scratches and general wear and tear..... It's just GOOD, man
And oh, boy, the Elites in this game have such ornate, elaborate armour, just as it should be!
Close second would be the design and art style of Halo Wars 2. The Blur cutscenes add a gorgeous realism.... and most of the cutscenes look dark or mysterious, which is exactly the kind of aura that game deserves. It fits well!