From your point of view people that played FFI first like me and were blown away by it would never like another FF as much as the first.
Then people that played the fourth and were amazed by it would have a similar experience.
So on.
I played three Final Fantasies before FFVI and I claim it the best. It's not my personal favorite but I consider it better overall than any of the others in the series.
I'm sorry, but from my point of view this appears to be another person who had FFVII as their first Final Fantasy complaining about people who think FFVI is better.
I think over-rating comes from an outsider's perspective. Everyone looks for different things in games and enjoys different things in games. Someone who found those things in FFX is going to consider the opinion of someone who found what they like an FFIV over-rated.