Post 8, hm? Do you mean numeraled titles only? If so, then 9, no question. If you prefer, or simply enjoyed, classic Final Fantasy, you will probably like Final Fantasy IX way more than any of the others, post FF8.
Final Fantasy X is an easy second choice, assuming you aren't interested in the Tactics series. It has a positively gripping story, and a curiously satisfying skill/level system (except for Kimahri... poor giant lion dude, starting basically trapped inside four Lv.4 keyspheres for most of the game in exchange for first access to Ultima... poor guy...).
Final Fantasy III, though not technically "post-8", only released to the West recently, and in a form "updated" from its NES days, so I suppose technically that counts as "new". (In fact, the DS title just got a Steam edition.) It's a curious game, and it's obvious that Final Fantasy V learned from and improved upon 3's features and systems a great deal, but it's nice to see the progenitor of the "modern" Job system. It's not bad, by any stretch, just maybe not as good as the SNES-era titles. If you really loved 5, you should play 3. If not, maybe play 9 or X first.
Too bad there is nothing after X, but there was a weird spin-off game that may be worth your time. They called it Final Fantasy: XII. The gameplay is... weird, more akin to Monster Hunter or Fallout than Final Fantasy (and that subtitle makes absolutely no sense), but a lot of people liked it. At least the story was interesting, full of twists and turns somewhat similar to the absolute classic, FF4.