Whatever you do don't get 8. The first 1-2 disks are pretty good, but overall its better if we just dont talk about that game >.>
If you want one that's most indicitive of the rest of the series then I'd say VII is your best bet. I personally found 7 rather boring though, as the first disk really drags at times. Of the ones listed I'd say 9 is my favorite because of the great story/character, but of course that's just my opinion. I've never played VI also, so I can't really say how good that is.
--
This is my signature.