There has never been a mainline FF that was not a least good (FF V and XIII were just good games to me), the problem is when multiple FF games are masterpieces (IV, VI, VII, X) it makes the "good" games look worse than they are to most. I think XV can be great (8, 9, 12) quality and with the slim pickings right now for JRPGs I will take it but it won't be enough for some who want the next masterpiece.
End of 2009 Predictions (Set, January 1st 2009)
Wii- 72 million 3rd Year Peak, better slate of releases
360- 37 million Should trend down slightly after 3rd year peak
PS3- 29 million Sales should pick up next year, 3rd year peak and price cut