Calling XIII terrible is not only an insult to the word but to kind on the game as well.
For me, if X and X-2 were the coffin and the lid for the Final fantasy series then XIII is the nails.
Final Fantasy X was still good FF when it came to the essentials, turn-based combat, grid, a world full of secrets, and fun mini games. A little odd in art direction, but they were trying to re-define the visual look of the series in order to prevent it from getting stale -- they did the same thing during the transition from SNES to PlayStation (less Steampunk more Cyberpunk).
Of Tidus's character and the Yuna romance I offer no defense.
I never understood the adoration for FFVI, that was utter shit, at least FFV had a decent class system (for a jrpg) and IV was a bit charming, but good god, VI was just annoying
What, really? Think about it.
It had the biggest, most elaborate, and most interactive open world with the most treasures, side quests, and powers, a cast full of charming characters that felt like (1) they were part of the world they were trying to save (2) were actually friends with each other, the most and the most colorful set, scenario, and character designs on the SNES generation, with some scenarios allowing you to control several parties at once in order to advance. As if that wasn't enough, they allowed the villain to succeed in his master plan of destroying the world and becoming a god; the only time in the Final Fantasy series where a villain makes that kind of blow against the good guys. As a consequence of that choice, the biggest and elaborate open world in the series was cracked to pieces like an egg and stuffed to the brim with
even more treasures, side quests, and powers (when it already had -- and still has -- the most).
In short, ambitious and successfully implemented narrative and game play design decisions.