I ain't played 'em all to be fair.
1 - VII
2 - VI
3 - IX
4 - I or X
5 - I or X
6 - XIV
7 - VIII

The correct order starts with Tactics, there is no other choice that is acceptable. Followed by VI, VIII, IV, V and X but that order can be moved freely around to taste

1 - VII
2 - I
3 - V or IV (unsure which)
4 - IX
5 - XV
6 - XII
7 - VIII
8 - X
9 - II
10 - III
11 - 11 and 14, never played them
12 - 16

Final Fantasy (Mainline/Numbered):

1.) IX
2.) VI
3.) X
4.) VIII
5.) VII
6.) XIV (Especially Shadowbringers)
7.) IV
8.) XII
9.) V
10.) XI
11.) I
12.) II
13.) XVI
14.) XV
15.) III
16.) XIII

Just my own personal order as a Final Fantasy fan since 1989. I've Played them all - even the MMOs, Mobile, Handhelds and Spin-offs - but that mainline/numbered titles are usually the focus of these lists and conversations. So this list of mine, does not include Remakes (like FFVIII Rebirth or FFIII Remake), Spin-ffs (Like TYpo-0, Final Fantasy Legends, Mystic Quest, World of Final Fantasy or Crystal Bearer)... or a few of the numbered titles that are not usually included here (like X-2, XIII-2, Lightning Returns, or IV: The After Years). They don't make the cut.

The Correct Order:

1. VIII
2. VI
3. VII
4. V
5. III
6. TYPE-0
7. IX

I'm playing it for the first time right now. and the truth is that the dialogues seem childish to me... the designs... those of ff 8 are better. The combat is a bit of a disaster, slow and the auto trance could have been left in manual. I have never wanted a character to die so much as Steiner, he is very heavy and repetitive in his dialogues.

for me it will always be 9, 6, 4,5 and 7 in that order. 12 was a lot of fun too on ps2. didn't care for the changes in the updated version

6, 7 and 9, close tie for them

That's just great. Somebody need to tell them the truth.

Sixth of course!!!