Let me start by saying that this is my personal observation from speaking to a relatively small sample set (okay, okay, chatting with my friends) and thus it is entirely possible that I am completely off base here.
I have noticed a strong trend of Final Fantasy players greatly preferring the first Final Fantasy they played over any subsequent game. I would guess that between a good dose of nostalgia and the inevitable comparisons one makes to the first Final Fantasy we play most people are significantly bias toward the first game they play. Certainly, for myself, FFIX is my all time favorite Final Fantasy (I picked a game at random off the Blockbuster shelf, what can I say), even after playing every other numbered game. There is a certain... emotional... attachment to the FFIX that has been lacking in the other games.
I suspect that besides being a genuinely good game, the fact that FFVII brought a significant portion of the current fans to the franchise may be part of why it polls so strongly. For a lot of people, they probably have an actual emotional attachment to the game as the first Final Fantasy they played.
I'd amend your argument and suggest that this happens with not the first but the first during adolescence.
I think during the formative years we create stronger bonds with things we associate with to create an identity for ourselves.
I'm 30 and my first Final Fantasy was, the first one, I didn't care for it. I played Final Fantasy Legends for Gameboy and I liked that a little better. I liked N/A FF II, then my favorite VI and subsequent VII. VIII was horrible, IX was eh, and X was too linear.
I went back and played the others but none of them resonated as much as VI and VII. And I think there are a lot of reasons both subjective and objective why that was the case.
Plus I think there's a big residual overlap from say, older brothers or sisters who played, who still have the game and whose siblings are playing them now.
But I truly think VI and VII are the best Final Fantasies of all time, and they'd strongly contend with the best JRPG's ever, although they'd have a tough time with games like Xenogears, Suikoden 1/2, Chrono Trigger, and Earthbound.