I personally stopped playing them after XII, so I picked all the generations from then back. I have a hard time not loving them all.

The "first generation" was carried by FFI. But for me, that's one of the most important games ever made.

The "second generation", like Spooooon said, is where FF really found it's footing. IV took most of the best things from the first three & brought everything together. V, even though most of us never played it until it was rereleased, was an important bridge between III and later FFs. VI brought the dynamic casting from IV and set a new standard for how big a FF could be.

The "third" is probably the most important. Not only did it bring many of you kids into the fray, it also elevated the standard for how epic a FF could be from VI. This generation really played w/ how you learn techniques and spells, that just leveling or buying them wasn't going to be enough. Another note, a lot of the weirdness in the scripts were cleaned up. I noticed the translations flowed better than the previous generations.

The "fourth" generation put it all together, brought the characters to life w/ voice acting and larger scale cut scenes. Final Fantasies at this point became almost playable movies.