While you may or may not agree on whether it's the series' best, Duke Nukem 3D was still a very important milestone in first-person shooter history. Released as the followup to a pair of platformers from 1991 and 1993, D3D starred the catchphrase-spewing, muscle-bound Duke Nukem on a quest to save the planet's women from invading aliens. Throughout your quest you'd visit contemporary-ish locales inspired by real-world cities; large, expansive outdoor areas; and sci-fi spaceships and moon bases.
Powering the game was the Build engine, which built (sorry) upon the foundations of the Doom engine, introducing features like destructible environments, letting you blow holes in walls and level entire buildings in first person. It helped D3D become one of the Big Three -- Wolfenstein 3D, Doom and Duke Nukem 3D -- responsible for popularising the FPS genre before actual, fully 3D game engines came to the mainstream.
GIF created using GalleyUK's video
For a game released on 29 January 1996, it was a hell of an experience, and it holds up well today, looking at the modern, updated Megaton Edition's popularity and reception. It's also a hell of a legacy to live up to, which is something 2011's Duke Nukem Forever certainly failed to do.
While looking for videos for the two GIFs in this post, I stumbled upon this 5-minute clip Lazy Game Reviews made for the anniversary. It's a good summary of what makes Duke 3D great, so for some additional nostalgia, you can watch it below. Of course, you could always just reinstall the game and play a bit. For old times' sake.
Top GIF created using Game Cinematics' video