I can't decide between IV and VI-- it'd be like asking a parent which child they love more. But the re-release of IV for GBA made it more than worth picking up again-- after you beat the game once, you unlock a new dungeon, and you can go back to earth and switch out party members to go fight Zeromus and check out the special dungeon. SquareEnix gave VI for GBA a similar treatment, with new Esper summons and a new dungeon. I'll be picking this one up.
There were good things about VII, but for music, story, characters and a whole slew of other factors, VI and IV blow it away.