FFVII is my favourate, it just had everything. VI was fantastic, but it just didn't stick in my mind as much. VII's plot was amazing if you let your imagination go, the characters were better IMO too. Materia kicks Espers ass anyday, it's much more customisable (as in it IS customisable).
Plus I prefer the mini-games of VII.

VI was great though, I should really play it again.