I'm all for Final Fantasy games, but the fact remains the two that got me hooked were 3 and 7.. 2 just wasn't it for the franchise. Which is why I'm voting for Metal Gear Solid in this round.
When I first played MGS, I was blown away. It takes a lot for me to get into games like this, because I'm just not big into action games by the way these are. I find it slightly difficult to have to use one analog stick to aim your gun, while using the other to move your character up, down, left or right - so you're on target. It gets annoying.
However, the storyline from the MGS games was amazing, and the gameplay was unbelievable. It was also more grown-up than I ever remember other games with similar play styles being. The controls weren't as hard as I've seen some before, and in the end.. MGS blew open a door to a whole new level of action games. So it takes my vote.