6. Metal Gear Solid
Back in the late 90s, everyone was talking about this game. The story, the stealth mechanics, the guards throwing grenades down vent shafts, the boss fights! I however, didn't know what the fuss was about and ignored it... until I played the demo. I then went out and bought it the next day.
So much about this game felt fresh and innovative. The stealth mechanics were amazing; constantly hiding behind cover, ensuring you didn't leave footprints in the snow; using camera blind-spots. It was all new. The level design was also superb using the keycards to gain specific access to new areas but still immersing you into the idea that this was a top secret facility that you were infiltrating. Even backtracking was fun!
It also had the unique Kojima style that rewarded clever and original thinking. The fight with Psycho Mantis is a clear example of this (switching controller input around and him reading you memory card!). Most games and designers wouldn't dare breach the fourth-wall for fear it would destroy immersion, yet in MGS it they breach the fourth wall on numerous occasions yet it actually improves immersion into the game world. Other examples of rewarding original thinking by the player include having the wolves piss on a cardboard box thereby enabling you to traverse their territory without getting attacked or using cigarette smoke to ID potential laser traps.
I haven't even mentioned the story which is probably the best in the series. It made you feel like you were truly in a movie and was a superb cinematic experience with a rollercoaster of emotions. It also had two seperate endings which always helps with the replay value and didn't go too outlandish as happened in MGS2.
In many ways, the gameplay and visuals haven't aged brilliantly (as most games of that generation), yet everything was crafted so perfectly to fit as a complete package very little compares. One observation of Twin Snakes on GameCube is that whilst they added moves from MGS2, they didn't fit with the core gameplay or design of the original. Taken as a whole package, it was probably the most well-thought out and polished game of the PS/N64 generation.