I would say:
- FFVII --> From this point on the community of FF really grew Intesively! You gotta play this one first.
- FFT --> Although not a REAL part of the FF series, I still highly recommend it (If you like turn based Strategy-RPGs)
- FFVIII --> The card game rocks ^_^
- FFV --> If you like the job system in FFT, you'll like this one too.
- FFX(-2) --> Also great game since it's graphics are great