A NeoGaf user by the name of Square2015 released some data concerning the sales of each Final Fantasy game. This data focuses specifically on first month sales on the mainline entries barring the original two, and a few spinoffs. Notably, Final Fantasy XII, XIII, and XV rounded out the top three spots with Final Fantasy Tactics selling the least with only 49,000 in its first month. This data can show how much the market has changed over time and what points in time in the series had the most hype surrounding them. You can check out the list below.

Mainline Series

FFIII Oct 1994:
#5 77,000 (2 wks)

FFVII Sep 1997:
#1 494,000 (4 wks)

FFVIII Sep 1999:
#1 579,000 (4 wks)

FFIX Nov 2000:
#2 331,000 (2 wks)

FFX Dec 2001:
#2 747,000 (3 wks)

FFX-2 Nov 2003:
#3 403,000 (2 wks)

FFXI Online [w/ PS2 HDD] Mar 2004:
#8 115,000 (2 wks)

FFXII Nov 2006:
#2 896,000 (4 wks)

FFXIII Mar 2010:
#1 1,322,000 (4 wks)

FFXIII-2 Feb 2012:
355,000* (4 wks)

FFXIII-LR Feb 2014:
165,000* (3 wks)

FFXIV-ARR Aug 2013:
#9 >115,000** (3 wks)

FFXV Dec 2016:
#2 1,573,000 (5 wks)



Spinoffs

FF Tactics Jan 1998:
#20 49,000 (1 wk)

FF Tactics Advance Sep 2003:
#2 203,000 (4 wks)

FF Crystal Chronicles Feb 2004:
#1 243,000 (3 wks)

FFVII -Crisis Core- Mar 2008:
#6 302,000 (2 wks)

FF Dissidia Aug 2009:
#7 130,000 (1 wk)