Final Fantasy IV combines a myriad of different settings, strong characters, a unique twist on the crystal motif, and an engaging battle system to help it rise above its fellow games. The story begins rather unexpectedly, with the hero committing a horrible act for his sovereign --- which you quickly learn is one among many -- and questioning his place and identity as a result.
Cecil is probably the most interesting of all Final Fantasy heroes as a result. He starts as a villain and the consequences of his moral weakness and inability to confront his king over what he knows is wrong leads to a host of other problems for himself and everyone he comes into contact with. There's Rosa's ordeal, of course, but also Tellah and Edward's sorrow -- and even Palom and Porom's suffering that all come from Cecil. His entire journey, not just his quest to be a paladin, is one of redemption, giving a unique atmosphere to the entire game and emphasizing the importance of the choices we make.
The rest of the characters are interesting in themselves, each with a story to tell. Tellah's has to be the most poignant, though, and there are few other games that have an old, frail man successfully taking on the main antagonist -- and then *spoiler* dying because of it.
This game was also the first to feature the Active Time Battle system, forcing players to strategize differently, especially when it comes to powerful spells and summons. Character strengths are as you would expect, too, with Rosa being weaker than her companions (though vital for success), Cecil as the all-around fighter and defender, and so on.
It's not short on difficulty either. Like IX, FFIV forces you to use certain party members, but this takes place throughout the story, and at times, makes each victory seem like it was hard fought and well earned. The segments with Edward are notable for that, though also the extended period of time where your only mage is Tellah, whose MP is purposely restricted for story purposes. All in all, Final Fantasy IV provides the most variety in gameplay, along with an emotional storyline touching on the essentials of human nature, for better and worse.