There series I have bought the most, and to this day it is still my favorite would be Final Fantasy.
The ones I own are:
I
II
III
IV
V
VI
VII
VII- Dirge of Cerberus
VII - Crisis Core
VIII - (Best game of all time in my opinion)
IX
X
X-2
XI (Yes, I play on Cerberus)
XII (Good game people, stop hateing)
XII - Revenant Wings
Tactics
Tactics Advanced
Tactics Advanced 2
FFCC
FFCC Rings of Fate
FFCC Echoes of Time
Which is basically all of them. Lol.
(I love SE, if you can't tell. I own most of their games).