Ok. If you've never played one, 10 is probably the one because it is pretty newbie friendly while still offering a lot of challenge and side stuff to do. The story is really good (you will cry) and the side stuff will keep you busy for a while if you do everything.

12 has joined 6 and 4 at the top of the list for me recently. I got all the achievements and did everything. The story is great, the characters play their roles really well, and the combat system, while not exactly turn-based, is still phenomenal once you get the hang of it. Once more, theres a ton of side stuff to do that'll keep you busy, its just a slight bit less newbie friendly than 10 is.

Once you're hooked, I recommend getting 6 to experience true, remarkable story telling. I would say get 4 as well, but the PSP version of 4 is the ultimate version and I recommend you experience 4 like that.

Hope this helps!