FFVII for many obvious reasons, but I would also have to say FFVI. FFVI is old-school Final Fantasy, and it has many of the elements Final Fantasy is known for. Also, it is relatively easy, so it won't frustrate newcomers. It's 2D though, and that might put off some people. But come on, if that puts you off, you will never truly experience what Final Fantasy is about, and where it all started.

So, FFVII and FFVI.