The Lovecraftian tone coupled with the Nine Inch Nails sound design and the classic FPS gameplay that popularized a lot of things such as rocket jumping and online multiplayer even more than Doom did, makes Quake a solid 90's classic game that people are still modding it and creating new content, and with engines like Darkplaces you can even install high resolution textures that really make the game feel a lot more modern. It really is their best game in my opinion and although it often falls under the shadow of Doom and Wolfenstein 3d, to me Quake just combines everything they had done at that point and perfected it. It was also a groundbreaking title in that it used polygon models instead of sprites, something that was really cutting edge back in 1996.