Well I finished Lightning Returns over the weekend and out of all three games, I think Final Fantasy XIII-2 was probably the better one. Lightning Returns ending is awful. Like Mass Effect awful or maybe worse. Final Fantasy XIII-2 had all the best things; lots of areas to explore, awesome music, one of the better enemies in the Final Fantasy line, awesome ending. Only complaints I really have for Final Fantasy XIII-2 is the cheesy dialogue and the battle system is kinda meh.
Lightning Returns does have the better battle system out of the three. It's much more fun than waiting for that ATB gauge to fill to do anything. They probably had to make that change, because besides a few times during the game, Lightning fights alone. The other problem, is Lightning Returns can be a very short game. If you do the main quest, you can complete the game very fast. Doing the side quests is almost pointless, besides increasing some stats and making gil. Once you complete enough to unlock the bonus dungeon, there isn't any real incentive to do any more (not even a trophy/achievement). New Game+ is nice I guess, and you actually unlock the ability to upgrade weapons and shields which is cool.
That ending, bleh. It also sucks shit there is only 4 areas to see in the entire game. Even the bonus dungeon is in one of those same areas. The final dungeon, again is mostly all in one of those areas. I will say there is a ton of crap to collect in regards to weapons, magic, shields, and GARBS (clothes). I think there is like 90+ garbs in the game, and no way am I going to go collect all those, so if you like to play dress up then you'll have fun with that. I also had to laugh that they had to write in a plot device for Lightning's wooden personality for this game, when it's exactly the same as the other two games.
I guess I would rate the games in the series as Final Fantasy XIII-2 -> Final Fantasy XIII (because it's the more of a traditional Final Fantasy type game) -> Lightning Returns. Lightning Returns is fun, but it doesn't give any sense of closure or anything on the story itself.