Released in the wake of Street Fighter 2 Madness in 1994, One Must Fall : 2097 was both one of the first fighting games to feature giant robots, and one of the first PC fighting games to actually be smooth and playable. It was a commercial release (as shareware) by Epic MegaGames, but the full version was released to the public as freeware back in 1999.
The theme is pretty straightforward - it is 100 years into the future, planets in the solar system have been terraformed and colonized, and a giant corporation called WAR basically runs everything using puppet governments. One of the most popular forms of entertainment are giant robot fights; a grand tournament is sponsored by WAR, and a number of different fighters enter seeking everything from power to retribution.
The best point about the game is simply that it is very well-designed and playable, holding its own with the best of the fighting games of the early '90s and still pretty fun to play today. The game also has a lot of depth compared to fighters of that period, however. You first choose a pilot, which determines your rankings in Power, Agility and Stamina; you then choose a robot, each of whom has different move sets and strengths of their own. In multi-player mode (conducted via hot-seat or modem connection), you have all the robots available to you, but in single-player tournament mode, you start with the most basic robot (the Jaguar) and win money via fights to upgrade and to purchase newer and better robot models. Tournaments are available indefinitely even after you have won them, so you can continue to build some powerhouse character which you can then import into multi-player battles.
The game also has an appealing presentation, blending an unusually anime vibe for an American game of the early '90s with an industrial feel and an almost 3D look for the robots. News reports recap each match after you have fought with a couple of screenshots of the bigger blows. The soundtrack is a bunch of pumping dance techno using digital .MOD files, not my personal cup of tea, but the menu theme seems to be one of the more beloved songs of PC gaming history, and I liked some of the level tunes.
The only major criticism I have with this one is that once you learn the moves and get used to the game's rhythms, and possibly upgrade your character some in tournament mode, the game really ceases to be any kind of a challenge. The computer AI is just not very bright from the beginning and never really gets any better, you just go up against robots with higher stats. Once you are upgraded enough you hit a point where you essentially are invincible unless you choose to throw a match. At that point, multi-player is about all you have left to amuse you.
One major stumbling block to the enjoyment of this one is that the programmers didn't plan for it to be run on modern machinery, and any computer from about 1996 onward will likely run it way too fast to be playable. The game runs well under DOSBox, but it does very little on its own to alleviate the speed problem. You can try Ctrl+F11 in DOSBox to adjust the speed downward while playing, but likely you will have to use a PC slowdown app like MoSlo to get it to where you can enjoy it (DOSBox is recommended anyway, though, just to get the native joystick support so that you can use a USB stick or gamepad to play, as well as making sound card problems less likely.)