Well as a fellow who owns every Final Fantasy game minus the online ones and the spin offs... I'd recommend 9 out of them. Personal favourite is and always will be 7 for three reasons, one is mainly nostalgia, I'll admit that one, was the first I played and I do have quite the bias towards it (got it for my 7th birthday), arguably the best most memorable score in the series and the storyline is pretty good... I know a lot of haters will hate on it but I did like the storyline, even coming back to it after years of playing it, I still enjoyed the story and the fact that Cloud isn't as much as a whiner that he gets heat for... also most if not all the characters have an arc (which is important).
However you've already played 7 so I'll go on to my second favourite in the series which is indeed 9. Its a good change of pace from 7 and as many will say here, the games aesthetic is closer to that of the original final fantasies. Also, I liked the main character.
The worst out the three you mentioned would go to 8. Its not a bad game but its certainly the worse out of the three mentioned. Don't let that put you off though, currently in an FF mood and I'm replaying this one as we speak

. The one thing I will note which is really out of the spirit of final fantasy is the level scaling, you'll actually want to avoid leveling up as it makes the game harder. Makes the game less grind tastic but Final Fantasy is always been a grind fest and that's why I love the series (aside from the fact that every and I do mean every Final Fantasy is scored to perfection).