I'd recommend ALL of them. But I'd start off with 5 and then go up until you reach 8.
I played most of the games chronologically as well and it's awesome to see the relations between the games (things that can be found in more than one game), PLUS how the games developed. ^^
If you just want to know my favorite from the ones you could possibly play: 6. It's the best FF ever. I've written a long explanation once, if you like, I'll search for it. But as it'd contain spoilers, you wouldn't want to read it. Just play it, then we'll see if you love it as well.
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