I LOVED FF 2's development system where everything had an experience bar. Each weapon became stronger in the hands of the character and stat depending on how frequently it was used. Aside from a few wrinkles in the system it was BRILLIANT and I want to see another RPG implement that system with more refined mechanics. I also give it props for having a good story or being the first serious storyline. I don't care what anyone says, it was simple and yet effective and it works well.
FF 4 had a great story, cast of characters, and good overall gameplay. Each character served their purpose, the story was great, and I really liked the whole twist with Golbez despite how "cliche" it may seem these days. Golbez is also one of my favorite classic villains, he really looked and played the part.
But the all time favorite classic FF is 6. I find little to no fault in the game. Excellent gameplay, meaningful and charming characters, a truly terrifying villain that induced that Heath Ledger joker feel in you, and an amazing story. Terra is still one of my favorite FF characters in the entire series and Kefka is just...Kefka. A demented human being that represents the epitome of all the negative aspects of humanity. Also extra points for being a villain that wins....to an extent, still destroyed the world though.