Final Fantasy XIII is (IMO) the most underrated game in the series, it has received an extremely unfair barrage of criticisms from gamers, mainly because of how it diverts from it's predecessors. If anything, FFXIII was a natural progression of the original vision of Hirobnu Sakaguchi who has strived to re-interpret the idea of fantasy
as a genre with each game.
I'll go on record as saying that the story of FFXIII is among the best in the entire series and Masashi Hamauzu excelled in his role as composer, creating a soundtrack that satisfied the void left by the departure of Nobuo Uematsu and also creating an entirely new feel that showcased Hamauzu's versatility. The gameplay is different, but just because the game is linear in nature, there is no need for people to shoot it down. I initially disliked the game, until I gave it another chance earlier this year and my opinion about it has done a complete 180.