I enjoyed X and disliked XIII. They share similarities but X was a hell of a lot more fun. Seiferalmasy already hit a lot of these points for me so i wont go into all of them.

X's battle system was far far more enjoyable. You were encouraged to use all the characters for their specific skills, but at the same time, characters weren't limited to specific roles. After reaching a certain part in the game, or when using the expert sphere grid, you could customize your characters, and without them losing their identity as overdrives and weapon types still set them apart.

The weapon upgrade system was superior to XIII. To customize specific abilities into weapons, you needed the corresponding items to use on them. Such a wide range of items and abilities encourage you to visit every corner of spira and fight many types of enemies. Where in XIII all you need is money to bulk buy a single component, therefore you simply need to visit one location and fight just one enemy. Not to mention that auto abilities run out in XIII making pretty much every accesory with an auto ability almost completley useless. Of course the final nail in the coffin for XIII concerning the weapon upgrades is the fact that you will never use your fully upgraded weapons because doing so will make 5 star battles 10 x harder to achieve.