The game that truely started the series, the series that can't be surpassed by any other in it's genre.
First of all, the game can be started and completed in a day. Most people would complain that that is a waste of money (and perhaps for back then it was) but from start to finish you are hooked, and you are in for one heak of a ride! it's like playing a movie, you watch then play, play then watch. But you don't mind it, because it works! You become immerced in the creative and luring storyline, while at the same time furthering the story personaly. To some, this game is easy, to others it is hard. For me I found just enough of a challenge for it to be "Just right". The boss battles are creative, and only games like Zelda could rival (or surpass) them. And of course: "This is a sneaking mission Snake" so if you don't crawl around not being seen, this game is going to be a lot harder than it needs be. The grathics are pretty good for PS1, and the sound is amazing.
Anyway it all boils down to this: If you can get your hands on it, play Metal Gear Solid.