Final Fantasy XIII: Great story and characters. Poor gameplay and design.
Final Fantasy XIII-2: Much better gameplay and design. But what the heck did they do to the story?
Lightning Returns: Final Fantasy XIII: I've only played the demo, but I already like the direction of it. It sounds like it has a clever design from the reviews I've read, the gameplay was fun despite being limited to the demo and the story needs what this game is offering, at least from what I've seen in the trailers. I look forward to checking it out whenever I have the money or the time. Won't be for a while, but I anticipate it.
Personally, I'm with those that say a sequel was unnecessary. Maybe Square Enix needed to prove they could still make games that play well but they completely screwed up the happy ending of FFXIII. They ought to have just made something new.
Oh and then there's Mog. I like the characters of this series a lot. Even the much-hated Vanille and Hope. But I absolutely hate Mog. And that's hard, to get me to hate.
I hate Mog. So much.