I love both FF and KH franchises - perhaps I just happen to like square-enix's various RPG styles, which includes Chrono Trigger, Chrono Cross and Secret of Mana.
From Final Fantasy, in order of personal preference:
VII
VI
VII Crisis Core
VIII
X
IV
VII Dirge of Cerberus
Tactics
I
IX
V
II
III
Dissidia (confusing game play!)
I started off playing FFVIII which I think is an excellent game because of the great flexibility afforded by the GF junctioning system. Got hooked, and from there I went backwards and played all the older games in the series, and then IX and X. The FFVII plot and all its characters is the one closest to my heart for some reason and Sephiroth my favourite villain of all time (must be all that angst).
From the Kingdom Hearts series, I liked I, II, BBS and COM equally well and actually find the card system in COM refreshing.
Sadly unacquainted with the latter games as I did not own a PS3. Perhaps the PS4 now....