Hostage Rescue 3 (HOSTAGE3.WAD)

Before banging out the now legendary Twilight Warrior, a somewhat realistic FPS conversion for Doom II to be played in the then budding Legacy engine, Sidearm Joe and Ziggy Gnarly made the Hostage Rescue series, culminating in Hostage Rescue 3. HOSTAGE3 was released back in 1998, a three-level minisode of sorts. The hook is the same, an attempt at simulating real covert ops activities in the Doom engine, starting out with a shooting gallery before moving on to two distinctly different missions. If you've played Twilight Warrior but haven't touched their salad days, you're not missing much, as MAP03 is a much easier version of Twilight's MAP04 ("Navy Seal") and MAP01 is barely a glorified shooting gallery. There's some cool stuff, though.

Twilight Warrior (TWILIGHT.WAD)

For awhile, Doom was perhaps the most realistic FPS kicking around, as laughable as that sounds. It was, though, and it's only natural that authors would try to manipulate the core aspects of Doom's gameplay in order to facilitate an experience even closer to conceptions of what a realistic first person shooter experience would  be. Two wild and crazy guys - "Ziggy" and "Sidearm Joe" - made a series of levels called Hostage Rescue that tried to emulate this experience, but their efforts came to a head in 1998 with the release of Twilight Warrior, published as Black Shadow Software with the aid of some other authors, some now quite famous in the Doom community. It's a ten-map episode for Doom II, originally designed to be played in what is  now an ancient version of the Legacy engine. Nowadays, any modern port that supports mouselook, jumping and DeHackEd should be able to run it.