I tend to like ones that change up gameplay somehow, and dislike ones that are just 'do more damage' etc.

To take paladin as an example, fight or flight is boring, requiscat transitioning you into a spell burst phase is interesting.

Manafication is one of my favorites because of how it operates with red mage mechanics. You want to use it when your mana bars are in a certain range, high enough to get the full benefit of the cooldown but not too high as to wast mana and end up with 100/100 to not get the extra benefit from verflare or verholy.