Out of all of the Doom releases over the past 26 years, Doom 64 is easily my most memorable.
Under the hood, it's pretty much the same old Doom you know and love - but with a HIGHLY OPTIMIZED atmospheric makeover (I can't stress this enough). It's revised dark tone is unique, and does not exist in any other versions of Doom. The levels are also completely new, with large maps that feature maze-like qualities and have a stronger focus on puzzle solving, rather than the classic "shoot your way through" approach.
The outstanding soundtrack is easily one of its strongest qualities. Gone is the high tempo 'Metallica-esque' synth riffs from the original. The Doom 64 soundtrack is wildly dark in its tone. Slow cries of demons and death are heard echoing in the distance while an ambient sustaining drone keeps you at the edge of your seat. Occasionally, waves of unnerving synth pads wash over you, or an aggressive organ give it a hellish, gothic feel.
The colour palette and textures are also refined with a more gloomy, sinister feel. Dark walls under a starlit skybox contrast with beautiful hues of purple and green lights.
Now, I realize Doom 64 was released 4 years after the first Doom, but when compared to the original, It really demonstrates how music, art and design can significantly impact the overall tone of a game.
As for the gameplay, i'm sure it has its faults - but the overall tone and experience was so good - I don't remember anything negative. It felt like a real nightmare, and I can't wait until its re-released for PC in 2020.