For me Final Fantasy equals a storyline like no other. One that can make even the butchest men cry XD
That's the core of the series. A story waiting to be told, usually very adequate to criticize our own worlds problems, like FF7 with polution and FFX with religion - the most noticeable of them

Final Fantasy is extremely well constructed and deep characters, both physically and mentally

FF is adventure in a world full of things to do, sidestories, mini games and Chocobos/Airships and a search for Cid