I am unable to narrow it down to one so...
Final Fantasy 7 (PS1)
Metal Gear Solid (PS1)
Another vote for FF7. Couldn't believe it when watsername kicked the bucket. I'd maxed the beatch out with weapons and materia as well! Did I have a previous save where I could get it back...nope!
I'll have to say Final Fantasy VII, when I play it, it just reminds me of a certain part of my life (11 years ago!). I love the way the musical score and story line are so memorable. Probably the game that got me into RPGs, I don't think I'd played one before it.