Having not ever played 8, I can only speak about 1-4 and 7. And honestly, I would play 1-4. Seriously. The first one, if nothing else, will give you an appreciation of how insanely hard the original RPGs were, and at the very least, introduce you to the monsters. I mean, if you can spend the huge amount of time needed to level up and beat the game, congrats.
Number two had some of the best music, imo, and it introduced extra people into your party. (The first featured only the hero.) It does tend to get a bit stale nearer to the end, though. I'd say maybe play it until you have both the Prince of Cannock and the Princess of Moonbrooke and a little past that.
Number three probably continues to be my favorite because it introduced the class system. You can create your own character, named whatever you want, and set him up as whatever class you want - soldier, fighter, pilgrim, merchant, wizard, etc. You can play it a bunch of times through with a different kind of party (much like FF1). You cannot change classes on the fly, though, like in FF5. (Except for when you can turn a Pilgrim or a Wizard into a Sage later in the game.)
Number four has an amazing story and timeline. You play different characters in different chapters and then it all comes together in the end, when they all meet up. I'm not a huge fan of the wagon, and also not having total control of the characters in Chapter 5, but eh.
I never got to play 5 or 6.
Seven was fine, but just got boring, imo. Plus I got myself stuck in one of the dark worlds or whatever you call them. (Long story.) The music is not as riviting, either, save for the town and battle themes.
That's my two(hundred) cents.