SUB-TERRANIA / Zyrinx / Sega Genesis
Sub-terrania is the very definition of a cool idea just totally blown up by kludgy controls, a brutal learning curve and general over-difficulty.
You remember the PC game series Descent, which was big back in the DOOM era? It was like DOOM, but you were in a spacecraft in a cave complex and could move in all directions. Sub-terrania is basically Descent in 2D - you play as a spacecraft piloting through an alien mine trying to rescue human POWs who are forced to work there.
The levels consist of a lot of tight areas to navigate through, and a good amount of enemies who are very aggressive about pursuing you. On your side, you have very limited firepower that rarely puts a dent in the enemies, and you are apparently also not in space as the laws of gravity are in effect and you'll drift down unless thrusting actively in some other direction. Bumping against anything drains your shields, it's possible to kill off the POWs you are trying to rescue with friendly fire, and if you don't constantly find fuel refills you basically get stuck and/or fall to your death.
The Hardcore Gaming 101 feature on this game claims it is easier than a Contra game, but I totally disagree, outside of Contra: Hard Corps and possibly Contra 4. I think this game is far more brutal and unforgiving, which is a shame because it also has a lot going for it. It has some neat level designs and mission objectives if you can push past the insane difficulty, the graphics are on the whole pretty decent and there's some neat use of limited FMV, and the music is some pretty good ambient stuff (from the composer who would later go on to score the Hitman series.) I think a lot of players are going to be put off too quickly to discover what it has in store, however.
* Gameplay Video