What Next Final Fantasy ?

IX, it has great characters and story.

try X. x3

I'd say FFX, it is probably my favorite. Admittedly, I haven't played 8 or 9.

Play V or VI , maybe IX

Definitely X, one of the best after that FF feels like eh
Or IX according to the creator

love more action. choose. xv xii x more tbs strategy

9 or 10

FFX is a good game, but they messed up the character faces on the Steam/PS3/PS4/Vita version. Original PS2 version ftw!

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Best modern FF? XII. But it's too late here for a long explanation why, sorry

9

Said by someone who has an entire leg tattooed with Final Fantasy female characters..
Go with XIII. It's greatly ported on Steam.
All of them are the most beautiful games ever created, of course, and 13 is not even in my top 5, but you asked for a "modern" FF and you played some of the best "old" ones. Go with it, trust me :)
From those you put into your poll, from your perspective, I'd go for them in order - 10, 12, 15.