well i guess there are a few reasons:

1. it was the first RPG i played, and im very glad it was this one.
2. the storyline was enormous.. you could make a game of midgar alone.. but thats just the start.
3. battle system was fast, and materia raising was easy enough.
4. story line was interesting, although a bit classic, it had its twists and turns that made you stick along for 3 discs.
5. all the characters were well thought out.. all had their backgrounds and low points in the game.
6. i actually bothered myself more on the shady guys in FF8 then the puppets that you control in FF7.. it just made more sense..

i could go on.. but this would be enough